The invention relates to the technical field of railway detection, and particularly relates to an appearance detecting system of a vehicle-mounted transponder. An image collecting device is installed on the central line position at the bottom of a vehicle body and is used for collecting image data of the transponder in real time according to a control signal of the controller; the controller is used for emitting control signals to a plurality of image collecting devices according to image collecting parameters input by a storage display server; the storage display server is used for acquiring environment information from an environment information acquisition device and acquiring image collecting parameters matched with the environment information according to the environment information. By the system of the embodiment, appearances of transponders of high-speed rail lines and common-speed lines can be detected, and the situation of labor and finance consumption in the prior art is prevented.