The stress-strain relationship for expansive soil is associated with water content changing. Through the laboratory test of the relationship between the deformation, strength, expansion parameters of the expansive soil and the water content, a expansive soil elastic-plastic constitutive model based on Mohr Coulomb criterion which has engineering practical value is proposed according to the humidity stress field theory. In accordance with fundamental code-run-principle of FLAC3D software, the basic principle of the secondary development program and detailed analysis combined with the expansive soil elastic-plastic constitutive model are proposed. The correctness of the expansive soil constitutive model is verified through the demonstration calculation of an expansive soil slope, combined with the water content field calculated by Seepage software.