The　deformation,　strength,　and　failure　characteristics　of　layered　composite　sandstones　(LCSs)　are　critical　issues　in　a　stability　evaluation　in　underground　engineering.　These　issues　were　examined　in　this　study　using　true　triaxial　experiments,　with　the　main　focus　being　on　the　influences　of　mechanical　contrast　(MC)　between　the　layers　in　a　three-dimensional　(3D)　restricted　stress　state.　The　overall　strength　of　LCSs　is　mainly　controlled　by　the　strength　of　the　weaker　sandstone　when　MC　exists　between　the　layers.　Failure　under　true　triaxial　stress　results　in　σ2-oriented　fractures　coexisting　with　σ3-oriented　fractures　within　LCSs.　The　overall　failure　of　an　LCS　test　specimen　exhibits　the　σ3-transverse　symmetric　fracture　mode,　i.e.,　σ3-transverse　symmetric　tension　near　the　σ3-loading　surface　and　σ3-transverse　symmetric　shear　in　the　middle　of　the　specimen.　A　new　strength　criterion　is　proposed　to　predict　the　true　triaxial　strength　of　LCSs　effectively.　The　study　results　provide　insights　into　the　cracking　morphologies　of　LCSs　with　mechanical　property　changes　in　individual　layers　under　true　triaxial　stress.　©　The　Author(s),　under　exclusive　licence　to　Springer　Nature　Switzerland　AG　2024.