With application of four designed simulation schemes,effects of the Taihang mountain terrain on heavy rain in August 1996 was explored by MM4 model,and the mechanism of the increasing precipitation in terms of some elements such as perpendicular motion,vaporflux etc.was discussed.The results show that the terrain affects obviously the intensity and location of precipitation cell,increases rainfall by 60%,and favors the upward current and vapor convergence.It is proved that the closer active terrain is,the better the model forecast will be and the higher terrain is,the more obvious precipitation enhancement effect will be.