Abstract: In order to explore the characteristics of the pressure fluctuation and the radial force during the startup of AP1000 reactor coolant pump,different numerical simulation schemes were carried out with the reactor coolant pump hydraulic model according the property changes and the parameters setting of AP1000 startup.Statistical method and spectral method were adopted to analyze the dimensionless pressure fluctuation intensity and radial force separately.The results show that the pressure fluctuation intensity of the guide vanes channel are stronger than that of the impeller channel and weaken as the temperature increases.The pressure fluctuation intensity of the impeller channel and pump case are weaker and change irregularly as the temperature increases.The amplitude of radial force fluctuation at blade frequency and twice blade frequency are weaker and change irregularly as the temperature increases.The amplitude of radial force fluctuation at five times blade frequency is the strongest and weakens as the temperature increases.Therefore when the reactor coolant pump runs at the temperature below the design operation temperature,it will cause the enhancement of pressure fluctuation intensity in guide vanes,which may enhance the radial vibration of reactor coolant pump.