Abstract: From the safety of reactor coolant pump,the numerical investigation of internal unsteady flow in reactor coolant pump of one 300MW nuclear power plant in China that is based on the calculation Navier-Stokes equation and standard k-ε turbulence model is carried out in this paper.Through the analysis of pressure fluctuation which locates at impeller,guide vane and inlet parts,it is concluded that the frequency of pressure fluctuation in the reactor coolant pump is exclusively governed by the blade-passing frequency.For the spherical shell pump,the maximum amplitude of pressure fluctuation decreases from inlet to guide vane but increases from hub to impeller edge except for the vicinity of impeller edge in the impeller part which follows the former mode.The comprehensive analysis of maximum amplitude of pressure fluctuation shows that the hydraulic parts could reduce the effects of pressure fluctuation,increase only a few vibrations to the whole unit and these are in favor of improving the safety performance of main pump unit.