Abstract: An one-dimension model is proposed simulating the regularity of moisture flow under rainfall condition.The relationship between shear strength and water content of expansive soil is studied with two different experi-mental methods.Based on research result the stability of expansive soil slope is studied considering the effect of rainfall.The result indicated that with the delay of rainfall the slope safety lowered accordingly and the unstable slid-ing plane is expanding to the shallow slide.The proposed method can avoid complex matrix adsorptivity measure-ment and can consider the unsaturated seepage.Therefore,it is generally suitable for simple engineering projects.