Abstract: Considering that the effect of shock waves on cosmic ray is simply produced by a disturbed region where the solar wind speed is raised and the diffusion coefficient is reduced. The numerical modeling on the base of solar modulation of galactic cosmic ray is performed. It is found that the decrease of diffusion coefficient is more effective than the increase of solar wind speed in producing the Forbush clecrease.The amplitude of Forbush decrease is getting smaller as it propagates outward. The Forbush decrease when two independent shock waves exist is simply the linear superposition of the two Forbush decreases produced as if the two shock waves exist separately.