The invention discloses a seat fatigue testing device which comprises a clamp. Multiple seats are installed in the clamp. Each seat comprises a cushion and a backrest, the backrest is vertically and fixedly connected to the left end of the cushion. An air cylinder mounting rack is arranged the right upper portion of the clamp and is located on the right side of the seats, a first air cylinder is installed at the positions, corresponding to the backrests, of the lower portion of the left side wall of the air cylinder mounting rack respectively, and a second air cylinder is installed at the positions, corresponding to the cushions, of the left end of the bottom surface of the air cylinder mounting rack. The left end of each first air cylinder is in linkage connection with the corresponding backrest. The bottom end of each second air cylinder is in linkage connection with the corresponding cushion. Each first air cylinder and each second air cylinder are respectively connected with an air source through a magnetic valve and a pressure regulating valve sequentially. The seat fatigue testing device can simply and reliably conduct fatigue testing on various seats including metro seats and meets the fatigue testing demands for multiple load modes, different load sizes and different load frequencies.