Abstract: In order to analyze the effects of seismic excitations on the running safety of a high-speed train passing a bridge, a train-track-bridge dynamic model considering seismic loads was set up based on the train-track-bridge dynamic interaction theory. As an example, a simply supported beam with a span of 32 m and bi-block track was modeled, and the dynamic responses of a high-speed train-track-bridge system under seismic excitations were analyzed. The results show that the effects of an earthquake on the dynamic responses of a high-speed train-track-bridge system is significant, and compared with the vertical response, the lateral response of a bridge is influenced greatly. The running safety and running stability of a train passing a bridge are affected due to seismic loads. When El Centro seismic waves with a normalized horizontal component of 1.0 m/s2 and a normalized vertical component of 0.5 m/s2 are subjected to, the reduction radio of wheel-load exceeds the safety limit if its running speed is greater than 250 km/h, and the derailment coefficient exceeds the safety limit if its running speed reaches to 300 km/h. Therefore, the influences of train speed should be taken into account in evaluating the running safety of a train passing a bridge under seismic excitations.