By using the improved iteration model for train-bridge coupled system, the analysis model of train-track bridge vertical random vibration based on Pseudo Excitation Method (PEM) was established. PEM was applied to transform the random track irregularities into the superposition of a series of vertical harmonic irregularities, while the iteration calculation was applied to solve the equation of the train-bridge coupled system. Taking a five-span simply-supported beam bridge passed through by a CRH2 high-speed train as numerical example, the accuracy and efficiency of the improved iteration model for train-bridge coupled system was verified. The results show that the calculation efficiency of the improved model is about 5 times that of the conventional model under the same calculation accuracy. The mean value and the root mean square (RMS) of the vertical random vibration response for train-track-simply supported beam bridge, which are caused by deterministic load and track irregularities, respectively, were calculated. It can be seen that the vertical displacement of the bridge is mainly controlled by the weight of the train, but the excitation of track irregularity has little effect on it. The vertical acceleration of the bridge and car bodies are significantly influenced by track irregularities, and the improvement of the track condition can effectively improve the riding comfort. The RMS of the train-bridge coupled system random vibration and the discrete of the vibration caused by track irregularities increase with the acceleration of train speed.