The invention discloses an automatic trimming machine of a sealing ring for a freight car braking system, and the automatic trimming machine is used for trimming the sealing ring. The automatic trimming machine is characterized by comprising a machine base (2), a driving mechanism (4), a transmission mechanism (6), a sealing ring fixing mechanism (8) and a trimming mechanism (10). Through the arrangement of a grinding disc (18) and the sealing ring fixing mechanism (8), the distance between the grinding disc (18) and the sealing ring fixing mechanism (8) is adjusted through a pressure-sensitive sensor (20), the sealing ring fixed to the sealing ring fixing mechanism (8) and a first grinding disc split body (21) are located in the most proper grinding positions, and trimming of the sealing ring is achieved by rotating the sealing ring fixing mechanism (8). Problems in the prior art are solved.