What is claimed is:1. A display panel, comprising:a base substrate comprising a display region and a peripheral region surrounding the display region, the base substrate comprising a first substrate layer, a third substrate layer and a second substrate layer which are sequentially stacked, a material of the second substrate layer comprising amorphous silicon,wherein the display region comprises a transparent display region, the transparent display region comprises a pixel region and a light transmission region, and a thickness of the second substrate layer located in the light transmission region is less than a thickness of at least part of the second substrate layer located outside the transparent display region;the display panel further comprising: a plurality of pixel units located in the display region of the base substrate, wherein the display region comprises a first pixel density region and a second pixel density region, a pixel density of the plurality of pixel units located in the first pixel density region is greater than a pixel density of the plurality of pixel units located in the second pixel density region, and the transparent display region is located in the second pixel density region.2. The display panel according to claim 1, wherein each of the plurality of pixel units comprises an organic light emitting layer, and a first electrode and a second electrode which are located at both sides of the organic light emitting layer,the display panel comprises a thin film transistor, and the thin film transistor is located at one side of the third substrate layer away from the first substrate layer,wherein the second electrode is connected with the thin film transistor, and the second electrode of the pixel unit located in the transparent display region is located in the pixel region.3. The display panel according to claim 2, wherein the second substrate layer located in the transparent display region comprises a first pattern, and an orthographic projection of the first pattern on the first substrate layer coincides with an orthographic projection of the second electrode on the first substrate layer.4. The display panel according to claim 3, wherein the second substrate layer located in the transparent display region further comprises an opening, and the opening is located in the light transmission region.5. The display panel according to claim 3, wherein a thickness of the first pattern is the same as the thickness of the at least part of the second substrate layer located outside the transparent display region.6. The display panel according to claim 3, wherein the second pixel density region further comprises a transition region located at a periphery of the transparent display region, the second substrate layer located in the transition region comprises a second pattern, and an orthographic projection of the second pattern on the first substrate layer coincides with the orthographic projection of the second electrode on the first substrate layer.7. The display panel according to claim 1, wherein the transparent display region is not provided with the second substrate layer.8. The display panel according to claim 1, wherein a thickness of the second substrate layer located at each position in the transparent display region is even, and a thickness of the second substrate layer located in the transparent display region is less than the thickness of the at least part of the second substrate layer located outside the transparent display region.9. The display panel according to claim 7, wherein a thickness of the second substrate layer located outside the transparent display region is even.10. The display panel according to claim 1, wherein a thickness of the second substrate layer located at each position in the display region is even, and a thickness of the second substrate layer located in the peripheral region is greater than a thickness of the second substrate layer located in the display region.11. The display panel according to claim 8, wherein the thickness of the second substrate layer located in the transparent display region is in a range from 1 nanometer to 3 nanometers.12. The display panel according to claim 1, wherein the display region is not provided with the second substrate layer.13. The display panel according to claim 1, wherein a material of the first substrate layer comprises polyimide, and a material of the third substrate layer comprises silicon oxide or silicon nitride.14. The display panel according to claim 1, wherein the base substrate further comprises a fourth substrate layer located at one side of the second substrate layer away from the third substrate layer, and a material of the fourth substrate layer comprises polyimide.15. A manufacturing method of the display panel according to claim 3, comprising:forming the first substrate layer;forming the third substrate layer on the first substrate layer;forming a second substrate material layer on one side of the third substrate layer away from the first substrate layer; andpatterning the second substrate material layer located in the transparent display region by using a mask plate to form the first pattern,wherein the mask plate is a mask plate for forming the second electrode in the transparent display region.16. A display device, comprising:the display panel according to claim 1; anda functional component, located in the transparent display region and at a side opposite to a light emitting side of the display panel, the functional component being configured to emit or receive light passing through the transparent display region.17. A display panel, comprising:a base substrate comprising a display region and a peripheral region surrounding the display region, the base substrate comprising a first substrate layer, a third substrate layer and a second substrate layer which are sequentially stacked, a material of the second substrate layer comprising amorphous silicon,wherein a thickness of the second substrate layer at each position in the display region and the peripheral region is even and not more than 10 nanometers;the display panel further comprising: a plurality of pixel units located in the display region of the base substrate,wherein the display region comprises a first pixel density region and a second pixel density region, a pixel density of the plurality of pixel units located in the first pixel density region is greater than a pixel density of the plurality of pixel units located in the second pixel density region;each of the plurality of pixel units includes an organic light emitting layer, a first electrode and a second electrode which are located at both sides of the organic light emitting layer, the display panel includes a thin film transistor, and the thin film transistor is electrically connected with the second electrode, and the second substrate layer is located at a side of the second electrode away from the organic light emitting layer.18. The display panel according to claim 17, wherein the thickness of the second substrate layer is in a range from 1 nanometer to 3 nanometers.19. A display panel, comprising:a base substrate comprising a display region and a peripheral region surrounding the display region, the base substrate comprising a first substrate layer, a third substrate layer and a second substrate layer which are sequentially stacked, a material of the second substrate layer comprising amorphous silicon,wherein the display region comprises a transparent display region, the transparent display region comprises a pixel region and a light transmission region, and a thickness of the second substrate layer located in the light transmission region is less than a thickness of at least part of the second substrate layer located outside the transparent display region;a thickness of the second substrate layer located at each position in the display region is even, and a thickness of the second substrate layer located in the peripheral region is greater than a thickness of the second substrate layer located in the display region.