Abstract: In recent years, with the large number of new AC-DC transmission EMU and electric train put into operation in China, the phenomenon of low-frequency oscillation appeared in electrified railway, which has caused great harm to the railway system. At present, when studying the low-frequency oscillation of the vehicle-network system, only the influence of the prepared vehicle is considered. However, under the same power supply arm, there are usually running vehicle with high power. Therefore, the difference of the impedance characteristics of the vehicle under different power is analyzed by establishing the impedance model of the prepared vehicle(low power operation) and the high-power running vehicle, and the influence law of the high-power running vehicle on the low-frequency oscillation of the vehicle-network system is analyzed by using the impedance ratio criterion. Finally, the simulation model of traction network, prepared vehicle and the high-power running vehicle is built to verify the correctness of theoretical analysis.