Abstract: In order to decrease pressure fluctuation of the reactor coolant pump under different conditions, three different inlet diameters of short blade of the reactor coolant pump were compared using numerical simulation, and the results show that the change of flow or inlet diameter of short blade does not change the dominant frequency of impeller. The high frequency of the suction side of blade gradually decreases and the high frequency of the pressure face gradually increases with the increase of inlet diameter of short blade. The pressure fluctuation amplitude of different inlet diameters of short blade with small flow is large. The wave energy of each monitoring point in the low frequency region and the high frequency region on the suction side of blade is significantly larger than that of the pressure face in the design condition. The band width of the low frequency region and wave energy of the high frequency region of monitoring points near the suction side of blade significantly increase at large flow fluctuation. The fluctuation amplitude of monitoring points near the pressure face of the blade has a large increase. The pulsation amplitude of each monitoring point near the suction side of the short blade is significantly higher than that of the long blade. The comprehensive analysis shows that when the inlet diameter of the short blade is 0.72 D₂, the pressure fluctuation in a variety of conditions reaches minimum.