The methods for analysis of stability of slope are usually considered, developed and improved along the line of finite slice method. But the safety factor of stability is facing challenge in engineering application. Because of the state of original stress in soil is ignored and disfigurement of finite slice method is in existence at all time. In different conditions of soil consolidation, K₀, the lateral stress coefficient of soil state, has influenced the property of soil and excavation of slope. In this paper, in consideration of the state of soil original stress, the calculating formula for safety factor have been given with applying Coulomb's equation for shear strength of soil and Mohr - Coulomb rule. There are many examples of safety factor using the formula of this paper, which have been calculated and compared with finite slice methods selected from reference literatures. The formula have showed their clear concept, simple calculation and dependable result. They can be used in engineering.