Abstract: To explore the influence mechanism of pre-swirling characteristics on the rotation field of the nuclear main pump, the primary system of the "Hualong one" reactor is taken as the research object, and the correction strategy of impeller blade placement angle is proposed through numerical simulation analysis. The results show that the main pump inlet produces a reverse swirl, and along the flow direction,the regularity of swirls is enhanced. The negative pre-swirling becomes relatively stable near the impeller inlet. The helix angle of the pre-swirling ranges from −15° to 13°, and the average swirl 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, the leading edge flow separation of the blade was eliminated. At the same time, the blade outlet angle was adjusted accordingly, which improved the pressure energy conversion efficiency of the impeller.