Abstract: Aimed at the block system with small strain,large displacement and rotation,3-D simple deformable distinct element method(3SDEM) was presented,and it was an efficient numerical method for simulating mechanical behaviors of discontinuums.It was suitable to be used to analyze nonlinear,large deformation and dynamic problems due to its explicit finite-difference scheme and automatic contact detection method.First,the equation of motion and deformation was established basedon the Newtonian Laws.Second,under the small deformation condition,the kinematic equation of blocks could be decomposed into rigid motion and deformation.Third,based on several deformation modes,the deformation of blocks could be expressed by the combination of deformation modes,which could be decoupled under specifical conditions.In 3SDEM,each block was an element,avoiding the increase of grid points of the block as what happened in 3DEC.So the computation cost was less than 3DEC.At last,two numerical examples were calculated with 3SDEM,and the reasonable results and the good computation stability were obtained.