Abstract: Pump system with slanted extension shaft is one of the main pump systems in large low head pump station. In order to improve the flow pattern in the slanted outlet conduit and increase hydraulic performance of pump system, the three dimension turbulent flow field was calculated for the pump system with 20° shaft extension in a large low head pump station based on the Reynolds-averaged Navier-Stokes equations and standard k-? turbulent model. By the method of numerical simulation, flow fields in outlet conduits were analyzed and compared for 4 kinds of calculation schemes which were straight outlet conduit without rotation, S-shaped curve outlet conduit without rotation, straight outlet conduit with rotation and S-shaped curve outlet conduit with rotation. Based on the analyses, the cause of serious bias flow in the slanted outlet conduit was analyzed. According to the influence of length of middle pier on the bias coefficient and hydraulic loss of the slanted outlet conduit, the method of solving the bias flow problem was put forward, and the fittest middle pier length was chosen, and the hydraulic performance of the pump system was tested. The research results showed that there was serious bias flow in slanted outlet conduit when the length of middle pier was 14 m. Looking from the direction of downstream flow, the main flow was obviously skewed to the left side of the slanted outlet conduit, while a large range of vortex area appeared in the lower right part of the outlet conduit, and the discharge of the left hole was obviously than that of the right hole. There was no bias flow in the straight and the S-shaped curved outlet conduits when the flow at the conduit inlet with no rotation. There was no bias flow in the straight outlet conduit while there was obviously bias flow in the S-shaped curved outlet conduit when the flow at the conduit inlet with rotation. Therefore, the flow with larger residual circumferential velocity component at the guide vane outlet enters into the S-shaped curved outlet conduit in a spiral shape was the cause of serious bias flow in the slanted outlet conduit. The results also showed that the middle pier length had a significant influence on bias coefficient and hydraulic loss of the slanted outlet conduit. When the middle pier length was 14 m, the conduit bias coefficient and hydraulic loss were large. Along with the middle pier length increased, the conduit hydraulic loss decreased firstly and then increased, the conduit bias coefficient decreased gradually. When the middle pier length was 23.35 m, the conduit bias coefficient was smaller than 1, the discharge of right side was bigger than that of left side in the outlet conduit, the main flow was skewed to the right hole. The discharge from the left and right sides of the outlet conduit is basically equal when the middle pier was lengthened to an appropriate value. The slanted outlet conduit scheme with proper extension of the middle pier length had been verified by the model test of the pump system. The efficiency of optimal operation point for the pump system with slanted extension shaft reached 80.56%, the model test results were basically the same with the numerical simulation results. The pressure values on the left and right sides were very close at corresponding time and the pressure variation trends were basically the same. These model test results indicated that the flow pattern of left and right sides were nearly the same and the expected rectification effect was achieved. The slanted outlet conduit scheme whose middle pier length was 23.35 m had been recognized by the relevant engineering design institute and applied to the engineering practice.