Abstract: Finite element model is established to calculate the vertical basic frequency of PC (prestressed concrete) simply supported box-girder with common span. The allowable impact factor of girder is determined according to the principle of the bridge dynamic effect under the action of running train not greater than that of design value. Based on the moving load model, the impact factor of the simply supported box-girder with common span is determined under moving train. The lower vertical basic frequency limit of PC simply supported box-girder with common span is recommended as 400 km/h, through comprehensively analyzing the relationship between the dynamic response of the girder and the type of train, the operation speed and the girder vertical basic frequency. The research shows that adjusting the ratio of the vehicle length to the bridge span could avoid the resonance of the fundamental vibration mode, which is beneficial to vehicle running safety. The lower vertical basic frequency limit of PC simply supported box-girder with span of 24 m, 32 m and 40 m are recommended, which are 125/L, 155/L and 90/L, respectively, which could effectively reduce the dynamic response of girder caused by vehicle with 25 m length.