In this paper, a mathematical model for the mid-span deflection of the beam bridge is given[3].A parametric SD oscillator[3-4] with time-dependent stiffness is proposed to explore the dynamics behaviour, which is caused by vehicles passing through the bridge successively.The special function of stiffness is used to simulate the impact from the moving load of the bridge.The movement of SD oscillator with time-dependent stiffness represents the mid-span deflection of the beam bridge under the moving load.