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.　©　(2011)　Trans　Tech　Publications.