Vibration analysis of steel truss bridge is required by promotion in the speed of train. The train and the truss girder bridge are coupled together as one composite system. Truss girder bridge is idealized as an assemblage of finite truss element. The vehicle space vibration model of two-stage suspension is used, which has 26-degrees of freedom. The equations of the train and truss girder bridges time varying system are set up by using the principle of total potential energy with stationary value in elastic system dynamics and the“set-in-right-position”rule for forming structural matrices. This method is more convenient than the finite elements. The vibration responses of train and bridge are calculated when the trains pass through a truss girder bridge at different speeds. The results show that the bridge has sufficient lateral and vertical stiffness.