The invention claimed is:1. A detector with a detachable antenna, comprising:a detector body, has a first connecting part, wherein the first connecting part is equipped with a signal receiving end;a connecting piece, wherein the first end of the connecting piece is detachably connected to the first connecting part; andantenna, wherein the antenna is provided with a second connecting part, wherein the second connecting part is provided with a signal output end, and the second end of the connecting piece is detachably connected to the second connecting part to make the signal output end contact and electrically connected to the signal receiving end.2. The detector with a detachable antenna according to claim 1, wherein the second connecting part comprises a sliding rod and a protruding structure protruding from a surface of the sliding rod, the connecting piece is provided with a sliding hole, an inner wall surrounding the sliding hole is provided with an blocking protrusion, and when the sliding rod is inserted into the sliding hole the protruding structure resists against the blocking protrusion.3. The detector with a detachable antenna according to claim 2, further comprising a sealing ring, wherein the sealing ring is sleeved on the sliding rod and clamped between the protruding structure and the blocking protrusion.4. The detector with a detachable antenna according to claim 2, wherein the connecting piece is provided with a connecting groove communicated with the sliding hole, and the first connecting part is configured to be inserted into the connecting groove to have the signal output end to contact and be electrically connected to the signal receiving end.5. The detector with a detachable antenna according to claim 4, wherein an inner wall surrounding the connecting groove is provided with a first connecting thread, the first connecting part is provided with a second connecting thread, and the first connecting thread is configured to be threaded to the second connecting thread.6. The detector with a detachable antenna according to claim 1, wherein the signal output end is provided with a first electrical contact of a concentric circular shape, the signal receiving end is provided with a second electrical contact corresponding to the first electrical contact, and the first electrical contact is configured to be electrically connected to the second electrical contact.7. The detector with a detachable antenna according to claim 1, wherein a handle is provided on the detector body for a user to grip and use.8. The detector with a detachable antenna according to claim 7, wherein the handle is provided with an uneven grip portion, which is used to fit the user's finger portion.9. The detector with a detachable antenna according to claim 1, wherein a control board is provided inside the detector body, and the signal receiving end is electrically connected to the control board, and the control board receives a signal transmitted by the signal receiving end.10. The detector with a detachable antenna according to claim 9, wherein a battery is provided inside the detector body, and the battery is electrically connected to and supplies power to the control board.11. The detector with a detachable antenna according to claim 9, wherein a pointer meter is provided on an upper surface of the detector body and Is electrically connected to the control board, the control board controls rotation of a pointer of the pointer meter based on the signal transmitted by the signal receiving end.12. The detector with a detachable antenna according to claim 10, wherein an indicator light is provided on an upper surface of the detector body and is electrically connected to the battery, and the indicator light is configured to indicate remaining capacity of the battery.13. The detector with a detachable antenna according to claim 9, wherein a switch is provided on a surface of the detector body and is electrically connected to the control board and configured to turn on or off the detector.14. The detector with a detachable antenna according to claim 9, wherein the detector body is equipped with a sound playback device electrically connected to the control board, the control board controls the sound playback device to play sound based on the signal transmitted by the signal receiving end.15. The detector with a detachable antenna according to claim 14, wherein an upper surface of the detector body is provided with a sound through-hole corresponding to the sound playback device.16. The detector with a detachable antenna according to claim 9, further comprising an adjustment switch electrically connected to the control board, and the adjustment switch is configured for the user to switch a signal frequency that the antenna can detect.17. The detector of the detachable antenna according to claim 1, further comprising a protective sleeve arranged at an end of the antenna.18. A detector assembly, comprising a detector with a detachable antenna and a pipeline endoscope, wherein the detector with a detachable antenna is configured to detect the pipeline endoscope, the detector with a detachable antenna comprising:a detector body, has a first connecting part, wherein the first connecting part is equipped with a signal receiving end;a connecting piece, wherein the first end of the connecting piece is detachably connected to the first connecting part; andantenna, wherein the antenna is provided with a second connecting part, wherein the second connecting part is provided with a signal output end, and the second end of the connecting piece is detachably connected to the second connecting part to make the signal output end contact and electrically connected to the signal receiving end.