In order to investigate the abnormal collision fault of piecewise stiffness spring-damper coupling in the transmission system of a diesel generator set during the startup，a dynamic model of the transmission system considering coupling collision was established. Based on the model，the fault mechanism and diagnosis method were studied，and the scheme to eliminate the fault was proposed. First，the frequency characteristics of measured torsional vibration signals of the transmission system during the startup of the unit were analyzed. Combined with the torsional vibration modal characteristics calculated by the free vibration of the transmission system，it is confirmed that the collision occurs between the driving and driven ends of the coupling when the unit passes the critical speed during the startup. Next，the stress of the key part(intermediate plate) of the coupling during the collision between the driving end and the driven end of the coupling was further studied. It is found that the maximum stress of the intermediate plate of the coupling reaches 325MPa，which exceeds the fatigue limit of 225MPa for the intermediate plate material，resulting in the fracture of the intermediate plate of the coupling. Then，a dynamic model of transmission system considering coupling collision was established，the mechanism of abnormal coupling collision fault during the startup of the diesel generator set was studied，and the influence factors such as stiffness and damper were analyzed. Finally，a scheme of replacing the original coupling was proposed and verified by experiment，and the problem of abnormal collision fault of coupling was solved.