Given the significant randomness of vehicle-bridge dynamic response for higher-speed railways, this study aims to explore the characteristics and probability distribution of dynamic response of a 400 km · h^-1 train passing through a bridge. A vehicle-bridge coupled random vibration model is established based on the pseudo-excitation method and the whole-process iteration method, and its validity is confirmed through comparison with the simulation results of Monte Carlo method. Based on this model, the time-frequency distribution laws of safety and stability indices of the train running at 400 km · h^-1 are analyzed, and the random characteristics of vehicle-bridge dynamic response under higher speeds on simply-supported beams with different fundamental frequencies are studied. The results show that the statistical values of vehicle-bridge response vary with time, showing typical non-stationary characteristics. The dynamic coefficient of the bridge is mainly controlled by the arrangement of train axle and the wheelbase, and is only slightly affected by the random excitation of track irregularity. Under resonance conditions of simply supported beam, the wheel load reduction rate increases significantly with the increase of speed, and the carbody vibration acceleration is insensitive to the resonance response of the simply supported beam. The fundamental frequency of the simply-supported beam has minor effect on the wheel load reduction rate and carbody vibration acceleration, whereas the randomness of the track irregularities has a significant effect on the vertical vibration acceleration and the wheel load reduction rate of the bridge.