Basic characteristics for development of surface cracks and state of slip surface during landslide disaster is studied, and relationship between surface cracks and internal fracture state is established. Continuum-based discrete element method(CDEM) is briefly introduced. Through simulation of specific and typical landslides with CDEM, relativity of strength and surface cracks is discussed and maximum number of surface cracks is found. Maximum number of slip surface cracks is defined as the number of cracks on slip surface which is already running through. The concept of fracture degree of surface and fracture degree of slip surface are proposed, which are defined as the ratio of current number of cracks to maximum number of cracks on surface and slip surface respectively. Numerical experiments show that there is a specific corresponding relationship between fracture degree of surface and state of slip surface, which could directly describe the risk of a geological body. Result shows that fracture degree does not rely on mesh division when calculation accuracy is guaranteed, which indicates that it is feasible to use this new method based on fracture degree and CDEM to evaluate the risk of a landslide.