A three-dimensional, viscous, steady CFD analysis is performed to investigate turbine flow field and performance of underwater vehicle by solving RANS-Navier-Stocks equations and RNG turbulence model. The flow detail and loss in turbine are studied for on-design and off-design conditions. The effect factor on turbine efficiency under off-design condition is analyzed. The results show that shock wave and flow separation occur in turbine under on-design condition, flow losses are caused by clearance flow in turbine. The turbine performance is degraded rapidly due to higher flow loss in nozzle as the back pressure increases under off-design condition. The predicted turbine efficiency agrees well with the test data.