A　series　of　undrained　cyclic　torsional　shear　tests　and　monotonic　torsional　shear　tests　are　carried　out　on　Fujian　standard　sand　and　Hutuo　River　fine　sand　under　different　initial　static　pore　water　pressures　by　using　the　hollow　cylinder　torsional　shear　apparatus　in　order　to　emphatically　discuss　the　effects　of　the　initial　static　pore　water　pressure　on　the　development　of　the　excess　pore　water　pressure　and　undrained　shear　strength.　The　experimental　results　show　that　the　initial　static　pore　water　pressure　has　a　significant　effect　on　the　development　of　the　excess　pore　water　pressure,　which　affects　the　static　and　dynamic　shear　characteristics　of　sand.　Specifically,　during　the　undrained　cyclic　shear　process,　the　greater　initial　static　pore　water　pressure　leads　to　the　faster　development　of　the　excess　pore　water　pressure　and　deformation.　During　the　undrained　monotonic　shearing　process,　the　greater　the　initial　static　pore　water　pressure,　the　greater　the　negative　excess　pore　water　pressure　during　the　sand　dilatancy,　which　significantly　increases　the　strength　of　the　sand.　Based　on　the　test　results,　the　mechanism　of　the　influences　of　the　initial　static　pore　water　pressure　on　the　excess　pore　water　pressure　and　static　and　dynamic　shear　characteristics　is　preliminarily　discussed.　According　to　the　results,　the　influences　of　the　initial　static　pore　water　pressure　on　the　anti-liquefaction　strength　of　sand　should　be　fully　considered　when　we　investigate　the　static　and　dynamic　shear　characteristics　of　soil　(partially　saturated　soil)　below　the　groundwater　table,　especially　the　liquefaction　problem.　In　laboratory　tests,　the　initial　static　pore　water　pressure　(back　pressure)　should　be　determined　according　to　the　depth　of　underground　water　level　where　the　sand　is　located.　©　2019,　Editorial　Office　of　Chinese　Journal　of　Geotechnical　Engineering.　All　right　reserved.