Abstract：The relationship between principal stresses in the plane strain state is the stress-strain relationship of soil with a certain strain path. 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 elastoplastic theory and test data under the plane strain condition，it is assumed that the relationships between the principal stresses in plane strain direction and those in the other directions are bilinear. The parameters of bilinear function are determined by one-dimensional consolidation and failure state. The principal stress in the plane strain direction is expressed as a function of stress states and material properties. Substantially，the proposed relationship of the principal stresses embodies the characteristics of deformation of soils in plane strain state. The function of this relationship passes three key points，that is the original point(1，1)，the state point of one-dimensional consolidation(bc，Rc) and failure point(bPS，RPS). Compared with test results，the validity and practicality of the proposed bilinear principal stress function are testified.
