Through dynamic performance test on prestressed concrete simply supported beam of Xiaotang-river Bridge on the Suhuang railway under different speed and different train formation, the dynamic characteristics of the bridge and its response under speed-increase trains are studied. Results indicate that the lateral forcing vibration frequency of train is close to the lateral natural vibration characteristics of the bridge when trains running at 70 and 75 km·h-1, and the resonance occurs. Consequently, vibration restraining measure for PCT beam should give priority to increasing the lateral rigidity of beam. When train speed exceeds 60 km·h-1, the maximum lateral amplitude of PCT beam exceeds the safety 1imit with no exception. The values of the lateral and vertical vibration acceleration of PCT beam are under the safety limits regulated by Code for Rating Existing Railway Bridges. The lateral vibration frequency of PCT beam in mid-span is obviously on the low level and is only 40-50 percentage of the reference value. The vertical natural vibration frequency of bridge in mid-span is enough and the ratio of vertical deflection and span is lower than the ordinary value stipulated in Code for Rating Existing Railway Bridges. It shows that the vertical rigidity of the beam is enough. Different formations of passing trains have a great influence on the 1ateral amplitude. When train formation is C64, the lateral amplitude is the maximum. while train formation is C64K, the lateral amplitude of the beam is smaller.