Using　a　ring　shear　apparatus,　the　variation　in　the　residual　strength　characteristics　of　21　groups　of　saturated　clay　soil　samples　were　experimentally　investigated,　and　the　results　are　discussed　in　this　paper.　The　clay　soil　samples　had　different　plasticity　indexes　under　10　different　consolidation　conditions.　The　influences　of　shear　rate,　over　consolidation　ratio,　plasticity　index,　and　multi-stage　shear　mode　on　the　residual　strength　were　investigated.　New　shear　strength　variation　trend　data　and　a　corresponding　equation　for　shear　strength　calculations　as　a　function　of　plasticity　index　of　soils　under　large　deformation　are　presented.　The　experimental　results　indicate　that　1)　both　the　shear　rate　and　over-consolidation　ratio　have　a　negligible　influence　on　the　residual　strength;　2)　the　residual　strength　gradually　decreases　as　the　plasticity　index　increases;　3)　the　residual　strength　shows　a　significant　non　linearity　with　the　normal　stress　when　subjected　to　low-　or　high-levels　of　normal　stress;　4)　the　internal　friction　angle　of　drained　clay　decreases　as　the　plasticity　index　increases　under　large　deformation.　Finally,　a　formula　is　proposed　to　describe　the　relationship　between　the　residual　inner　friction　angle　and　plasticity　index.