When Electric Multiple Units (EMU) is running in high speed, the working function of yaw damper is closely related with the vehicle stability. When a malfunction occurred at yaw damper with different levels and combined with malfunction from multigroups of yaw dampers, the effects were analyzed, the theories of dynamics and generalized resonance of vehicle system were applied and its running state was simulated. Based on the size of remaining damping force of antiyaw damper after malfunction occurred, the vibration performance and vehicle dynamic index of either carbody or structural frame in malfunction were analyzed, and the antiyaw damper degree of malfunction was found which easily caused malfunction state of vehicle resonance and crisis of its stationary. A yaw damper malfunction identification method was raised of either single malfunction or the combination of multimalfunction, so as to identify and exclude further damage as soon as possible. The analysis results show that this method is feasible.