Abstract: An unsteady numerical simulation was performed for the effect of the unsteady flow from the cavity leakage on the turbine stage.LU-SGS-GE(lower upper symmetric Gauss Seidel Gaussian elimination method)implicit scheme and high precision high resolution MUSCL TVD(monotone upstream-centred scheme for conservation laws,total-variation-diminishing) scheme were used to solve 3-D Navier-Stokes equations.The dual-time step implicit scheme was used for unsteady calculation.The results show that the patch of the cavity leakage through the rotor passage is moved close to the suction surface owing to the effect of the secondary flow.As the path is convected downstream,it migrates radially towards midspan.With the leakage flow entering into the rotor,there is more unsteadiness on the root of the rotor than that of the mid-span,and the unsteadiness becomes weak as the leakage flow migrates downstream.