Abstract: The shear strength parameters c， have different exerting extent during the slope failure.Although the two safety factors fairly represent the differences of the strength reserve of c and ，its strength reduction are uncertainty，and there lacks a unified standard regarding to the measuring of the slope stability.In this paper，according to the Mohr-Coulomb yield criterion and the strength reserve principle，comprehensive safety factor was put forward; The relationship between reduction factors of c and  was obtained from the derivation of soil softening process; A user-defined program which expresses a non-proportional strength reduction method was compiled with the fish language in the FLAC3D to analyze slope stability based on different strength parameters.The main research results are as follows:①Comprehensive safety factor eliminate the uncertainty of two safety factors in slope stability evaluation.②The cohesion c plays a more important role than the internal friction angle  does in the clay slope destroy.③The improved strength reduction method takes a consideration on the soil deterioration process that the traditional strength reduction did not do.

Key words: Non-proportional strength reduction, Two safety factors, Comprehensive safety factor, FLAC3D