The invention discloses a trimming device for seal rings on a wagon braking system. The trimming device is characterized by comprising a seal ring fixing device (2), trimming device bodies (4), a connecting device (6), a limiting device (7), a first driving device and a second driving device (9). Provided with the seal ring fixing device (2) and the trimming device bodies (4), the trimming device is capable of automatically trimming through rotation of the first driving device, but a difficulty lies in positioning between the seal ring fixing device (2) and the trimming device bodies (4). The trimming device for the seal rings on the wagon braking system has the advantage that the difficulty in positioning between the seal ring fixing device (2) and the trimming device bodies (4) is overcome with the help of a linear light emitter (16) and a light receiving panel (14), so that a trimming effect is further improved.