By studying on rock mass discontinuities of Yanjinqiao Reservior right bank and analyzing discontinuity's geometric and mechanics property,with different types of discontinuities,random discontinuity's geometric parameters are obtained with the method of Monte-Carlo simulation and the certainty discontinuities are simulated with finite element software ANSYS to get effective method of discontinuity's simulation.The method could be not only deal with a problem of large quantity and small site random discontinuity's simulation one by one,but also be considered with deformation and strength for weakening factor.The results are near to practical situation and this method is effective.