What is claimed is:1. An ultra-thin speaker, comprising:a housing;a magnetic circuit system adapted to generate a magnetic field disposed in the housing;a vibration system comprising a cone assembly and a voice coil, wherein the cone assembly has a taper of less than or equal to 25°, a peripheral edge of the cone assembly is connected to the housing, and a middle part of the cone assembly has a through hole, the voice coil is fixedly connected at an edge of the through hole, the voice coil surrounds and is suspended around the magnetic circuit system, the voice coil is configured to be able to vibrate by an action of the magnetic field generated by the magnetic circuit system when a sound signal is introduced; anda dust cap assembly comprising a dust cap body, wherein a central region of the dust cap body is fixedly connected to the magnetic circuit system, and an edge region of the dust cap body is connected to the cone assembly and is configured to be able to expand and contract along with the vibration of the cone assembly,wherein the dust cap body comprises a fixing portion and a telescopic portion extending around the fixing portion, wherein the fixing portion is fixedly connected to a top of the magnetic circuit system, an edge of the telescopic portion is connected to the cone assembly so that the dust cap body covers the magnetic circuit system and the voice coil, and the telescopic portion is configured to expand and contract in response to vibration of the cone assembly.2. The ultra-thin speaker of claim 1, wherein the dust cap assembly further comprises a thermal insulation pad disposed between the fixing portion of the dust cap body and the magnetic circuit system.3. The ultra-thin speaker of claim 1, wherein the magnetic circuit system comprises a first magnet, a second magnet and a plate, wherein the first magnet and the second magnet are disposed having magnetic poles thereof positioned opposite to each other, the plate is disposed between the first magnet and the second magnet, and a magnetizing direction of the first magnet and the second magnet is parallel to a vibration direction of the voice coil.4. The ultra-thin speaker of claim 1, wherein the vibration system further comprises a damper connected to the voice coil, the damper having a loop structure, an inner edge of the damper being connected to the voice coil, and an outer edge of the damper being connected to the housing.5. The ultra-thin speaker of claim 4, wherein the voice coil comprises an outer bobbin, an inner bobbin, and a coil wound between the outer bobbin and the inner bobbin, with the inner edge of the damper being connected to the outer bobbin.6. The ultra-thin speaker of claim 5, wherein a conductive sheet is fixedly disposed on the outer bobbin, the coil is electrically connected with the conductive sheet, and circuit leads are disposed on the damper, the conductive sheet is electrically connected with the circuit leads, and the circuit leads are configured for electrically connecting with an external device.7. The ultra-thin speaker of claim 6, wherein the circuit leads are coated on a surface of the damper in a form of braided metal wires.8. The ultra-thin speaker of claim 5, wherein the outer bobbin and/or the inner bobbin are provided thereon with heat dissipation holes.9. The ultra-thin speaker of claim 1, wherein a taper of the cone assembly is less than 15°; and a height of the ultra-thin speaker is less than or equal to 10 mm.10. The ultra-thin speaker of claim 1, wherein the dust cap body is made of PU.
