In tests on dynamic performance of high-speed trains,it is found that there is an unfavorable speed interval for some vehicles,which would be harmful to the daily operation of the vehicle.By analyzing the relationship of vibration frequencies of the vehicle and its running speed,this paper finds that the unfavorable speed interval is caused by the vibration superposition of the natural frequency of the vehicle system with the frequency of external excitation.Taking some electric multiple units (EMUs) as examples,we proposed an approach to obtaining the unfavorable speed interval of vehicles.By analyzing the relation between vibration frequencies and the running speed of the vehicle,the natural frequencies of the vehicle system and the external excitations are distinguished.In the end,we suggest some measures to minimize the negative influences of the unfavorable speed interval,such as shifting frequency,reducing or eliminating external excitation.