Abstract: On the basis of the theory of three-dimensional flow, flow characteristics of 80LLW screw centrifugal pump system were tested and studied. By measuring axial velocity, radial velocity , tangential velocity and angular velocity under different conditions, their distributions were analyzed in detail. The results indicate that the flow inside the pump is spiraling, turning and non-symmetry flow. At last, this paper explains the reason of that phenomenon and points out its influences on the pump operating performance.