The influence of interstage leakage among impellers to the flow state outside the front and back shrouds of the multistage centrifugal pump was studied. The velocity distribution of inner flow and the value of axial force were measured by the experimental device of centrifugal pump. The results indicated that the rotation velocity of the liquid outside the front shroud was 0.5 times larger than the angular velocity of impellers, and the velocity outside the back shroud was 0.5 times less than the angular velocity. The pressure outside the front shroud was lower than the value outside the back shroud. The actual value of axial force was lower than the calculated. And the formula of the axial force should be modified. 