Abstract: The unsteady Navier stokes equations are solved to predict numerically the internal flow of single cavity labyrinth channel with depth to width ratios H/W=0 1,0 3 and 0 5.The numerical results demonstrate the oscillation of the flow structure and fluctuation of the exit leakage rate.It is shown that the seal efficiency is relevant to the amplitude of leakage fluctuation,furthermore,the sequence of H/W from 0 3,0 5 to 0 1 is also the order of both increasing seal efficiency and increasing amplitude of leakage fluctuation.Also,the complex physics,including motion and mergences of vortex,migration of separation bubble,bending and impact of jet,is discussed,and the influence of impact effects on the increasement of energy dissipation and decreasement of leakage are analysed.