Abstract: A brief analysis on the mechanism of slump failure of fissured soil (expansive soil) slope is given. By the qualitative conclusion of method of limit equilibrium and the finite element analysis, the simple analytical formula for solving the plastic zone is established. The plastic zone is brought into the analysis of slope stability, and the effects of various characteristics of fissured soil on slope stability are indicated. According to these effected factors, the new sliding surface diagram and model are established, and the results of which has fair agreement with the statistic data from observation. The reference curve for slope design are finally presented.