Abstract: To explore the influence mechanism of pre-swirling characteristics on the rotation field of the nuclear main pump, taking the primary system of the "Hualong one" reactor as the research object, proposing the correction strategy of impeller blade placement angle through numerical simulation analysis. The results show that the main pump inlet produces a negative pre-swirling along the flow direction. Near the inlet of the impeller forms a relatively stable negative pre-swirling. The helix angle of the pre-swirling ranges from -15 ° to 13 °, and the average helix strength is 3.5 °. The pre-swirling increases the inflow angle of the liquid. When the inflow Angle increases, the liquid flow direction deflects to the working surface, which aggravates the separation flow loss at the inlet of the working surface. According to the determined value 0.2 of inlet velocity-moment to adjust the impeller inlet blade placement angle, eliminating the leading edge flow separation of the blade. At the same time, change the blade outlet angle accordingly, improving the pressure energy conversion efficiency of the impeller.