The increase of water content will cause swelling deformation of expansive soil foundation.The swelling rate and pressure are raised linearly with water content.The change of water content of expansive soil foundation decreases linearly with depth within the atmosphere effect depth.The expansive rate is elliptic with overburden pressure.A formula of expansive rate is obtained.A calculation method for swelling deformation of expansive soil foundation is proposed with layer-wise summation method.The results of proposed method are identical with that of the model test,and the results of the code are on the large side.