Abstract: Lateral stability is a safety evaluation indicator of railway vehicles. Lateral instability happened to a type of EMU trains for several times. By failure chain analysis, three causes of lateral instability were identified. Based on the abnormal vibration of wheel-sets, axle box and frame as well as the line information, the wheel wear was identified as a main reason of lateral instability. Based on the transmission relationship of vibration from frame to beam of body, the decrease of damping value of the anti-snake damper or lateral damper was identified as a main reason of lateral instability. Based on the polygon feature frequency of frame vibration, the polygon wheels were identified as a main reason of lateral instability. The identification method of lateral instability was applied, which provided a reference for identifying the lateral instability reason of EMU trains.