The　nonlinear　strength　characteristics　of　rock　materials　under　3D　stress　states　are　studied　in　the　deviatoric　plane　and　meridian　plane.　The　effect　of　intermediate　principal　stress　and　hydrostatic　pressure　as　well　as　coupling　effect　of　them　are　studied　in　these　two　planes　respectively.　By　conducting　linear　interpolating　of　external　normal　vectors　of　octahedral　plane　and　spatial　mobilized　plane,　a　model　of　the　unified　strength　theory　is　proposed.　Each　kind　of　material　corresponding　to　a　shear　failure　plane,　then　the　unified　strength　criterion　is　established.　The　criterion　has　four　parameters,　all　of　which　have　obvious　physic　significance　and　it　owns　continuous　smooth　failure　plane　in　the　principal　stress　space.　It　is　verified　by　triaxial　compression　test　data　of　five　different　rocks　that　have　been　published.　Two　transformed　stress　spaces　are　proposed　based　on　the　unified　strength　criterion;then　the　criterion　is　transformed　into　Mohr-Coulomb　criterion　and　Drucker-Prager　criterion　in　the　new　stress　space　respectively.　Triaxial　compression　test　data　show　that　the　unified　strength　criterion　can　be　used　as　Mohr-Coulomb　criterion　and　Drucker-Prager　criterion　in　transformed　stress　space.