The characteristics of deformation and strength of soils under the plane strain condition can be simplified as a two-dimensional problem. How to easily and reasonably determine the value of principal stress in the direction of plane strain is the key point. Based on the Elastic-plastic stress-strain relationship for soil and test data under the plane strain condition, it is assumed that the relationship between principal stresses in plain strain direction and the principal stresses in the other directions is bilinear. The parameters of bilinear function are determined by one-dimensional consolidation and failure state. The principal stress in plain strain direction is expressed as a function of stress states and material properties. Compared with test results, the rationality of the proposed bilinear principal stress function is testified.