Abstract: Under the condition of rainfall, the decrease in shear strength of a slope is one of the important factors that lead to the slope instability. Infiltration of rainwater into the slope is a dynamic process, and the water content or saturation of soil will change continuously. Therefore, it is of great significance to study the shear strength of soil under dynamic environment for real-time evaluation of the stability of soil slope under rainfall conditions. Based on the strength theory of unsaturated soil, this paper takes the expansive soil in the Jianshanying area of Guizhou Province as the object, combines the shear test data to establish the fitting relationship between the shear strength index and water content of the expansive soil, and puts forward the shear strength formula with water content as the variable. Based on the secondary development technology of ABAQUS platform, USDFLD subroutine is compiled to import the shear strength formula of the Jianshanying expansive soil, and the influence of strength attenuation and gravity change caused by the change of soil moisture content on the stability of the expansive soil slope under the condition of rainfall are also explored. The results show that with the increasing water content of the expansive soil, the particles in the soil expand by the absorbing water, the particle spacing decreases, and the adhesion between the mineral particles increases gradually. Until the water filling suction of the aggregate space decreases, the adhesion decreases rapidly, and the shear strength of the expansive soil decreases generally, in which the cohesion increases first and then decreases, and the internal friction angle decreases. The secondary development technology of ABAQUS software can realize the relationship of soil parameters with the change of water content, which provides a new technical means for the finite element analysis of unsaturated soil slope under the condition of rainfall.