Abstract: To solve the matrices of stiff,mass and damping varying with time in vehicle-bridge vibration equations,the train and bridge were coupled together as one composite system.The vibration equations of vehicle-bridge time varying system were established by means of potential energy principle.The vibration responses of a continuous truss girder bridge were analyzed by using this method.The lateral vibration calculation results of a truss girder bridge are close to the testing values.This confirms the reliability of the method.