In the area of alluvial soft soil, the double layer subgrade with rigid upper layer and soft lower layer is common. In such subgrade, the stress diffusion phenomena will occur under vertical load. In this paper, the stress and deformation of such subgrade are studied by ABAQUS. According to the calculation results, the plastic zone of subgrade appears in the upper layer soil of the interface, and then, it expands up and down the interface with the increase of load. The plastic zone expands rapidly in the lower layer soil. Finally, the subgrade gets up to its ultimate bearing capacity when the plastic zone reaches the foundation bottom. This results provides an effective analytical methods to the engineering designers. It has practical meaning.