Back　pressure　saturation　method　was　commonly　adopted　to　improve　the　saturation　of　samples,　but　specific　provisions　haven＇t　been　given　for　the　magnitude　of　back　pressure　in　current　Chinese　standards.　By　employing　the　vertical-torsional　coupling　shear　apparatus　and　the　automatic　true　triaxial　apparatus,　the　monotonic　drained　and　undrained　shear　tests　were　carried　out　under　different　back　pressures　for　the　Fujian　standard　sands　(with　relative　density　as　30%,　58%　and　70%,　respectively)　and　fine　sand,　so　that　the　stress-strain　relationships　for　sandy　soil　and　the　development　law　for　excess　pore　water　pressure　under　different　back　pressures　can　be　analyzed.　The　test　results　show　that　the　back　pressure　has　an　obvious　effect　on　the　shear　strength　of　dilatant　sand　in　consolidated-undrained　condition,　and　with　the　increase　of　dilatancy,　the　effect　tends　to　be　more　evident;　back　pressure　has　no　significant　influence　on　both　the　undrained　shear　strength　of　shear　shrinkage　sand　and　the　shear　strength　of　sand　in　consolidated-drained　condition,　while　it　almost　has　no　influence　on　the　effective　shear　strength　indexes　of　sand.　According　to　this　study,　the　obvious　effect　of　back　pressure　on　the　shear　strength　of　dilatant　sand　in　consolidated-undrained　condition　may　be　attributed　to　the　fact　that　with　different　back　pressures,　the　bulk　modulus　of　gas-liquid　mixture　in　soil　shall　be　different　so　that　in　the　same　volumetric　expansion　or　in　the　same　volumetric　rebound,　the　induced　excess　pore　water　pressures　shall　be　different,　which　may　further　directly　affect　the　confining　pressure　and　shear　strength　of　sand.　During　the　shear　shrinkage　process　of　soil,　the　closer　the　gas-liquid　mixture　approaches　to　a　rigid　body,　the　less　the　bulk　modulus　depends　on　the　back　pressure,　so　that　in　the　shear　shrinkage　condition,　the　back　pressure　has　no　obvious　effect　on　the　stress-strain　relationship　and　shear　strength　of　sand.　In　the　consolidated-drained　tests,　no　matter　how　large　the　back　pressure　is,　there　is　no　excess　pore　water　pressure,　and　the　effective　confining　pressure　is　irrelevant　to　the　back　pressure　so　that　the　back　pressure　has　no　significant　effect　on　the　shear　strength　of　sand　in　drained　condition.　©　2016,　Editorial　Office　of　China　Civil　Engineering　Journal.　All　right　reserved.