The retaining wall structures have always been designed for supporting talus in karst mountainous area. Because of large particle size of talus， its mechanical properties are very different from fine grained soil，However，most theories and experiments based on continuum theory took the fine grained material such as fine sand as research object， while less research on active earth pressure of large particle size such as talus was performed. Aiming at these problems， the discrete element method was used to simulate the active earth pressure of fine grained soil in translational displacement mode of retaining wall as well as talus in different particle size. The rule of active earth pressure with particle size was obtained through summarizing the results， and the point of active earth pressure correction formula which considered particle size was also deduced.