Because rainfall seeping makes losing stability of unsaturated soil slope，and arouses great loss to production and human being safety，the stability of unsaturated soil slope has been researched by many scholars recently．This article mainly uses the model for the prediction of shear strength with respect to soil suction，developed by Vanapalli and Fredlund to formulate rainfall seeping how to affect the stability of unsaturated soil slope．Firstly，volumetric water content of unsaturated soil slope changes with rainfall duration，and effective saturation changes with its volumetric water content．Secondly，soil volume weight changes with its volumetric water content．Thirdly，matric suction also changes with its volumetric water content．According to these causes，this article researches how much they make the contribution to the minimum safety coefficient respectively．At last，these factors roundly considered，this article gets the rule of minimum safety coefficient of unsaturated soil slope with rainfall duration that is minimum safety coefficient gradually increasing firstly，then decreasing that is composed of two sectors，first is slowly decreasing，then is fast decreasing after some value．