The theory foundation of discrete element method (DEM) is studied in this paper firstly. Dynamic discrete element calculation principle and method, dynamic loading input and boundary conditions are introduced and discussed before application. Rock mass stability under earthquake load by means of a large hydropower underground powerhouse cavern is calculated then. The displacements and velocities of rock mass around powerhouse cavern are monitored. And the result shows that DEM can be used to solve structure-controlled failure under dynamic load. The method and idea can provide guidance for underground cavern anti-earthquake design.