In order to study internal flow variation when centrifugal pump cavitations occur, three-dimensional modeling was used for centrifugal pumps internal flow through three-dimensional software Pro/E. Based on the Reynolds-averaged N—S equations with the RNG k—εturbulence model and SIMPLEC algorithm, the full-cavitation model was used considering effects on cavitations when the gas was not dissolved in the water. By using the computational fluid dynamics software CFX, the numerical simulation calculation of vapor-liquid two-phase turbulence within whole flow passage of centrifugal pump was conducted and the laws of the unsteady flow when centrifugal pump cavitations occur were analyzed. The results show that within this range from the newborn to the critical cavitations NPSH, the gas volume fraction mainly begins from dimensionless radial position 0.2, and the gas volume fraction density correspondingly increases with the lower of NPSH. Affected by asymmetry distribution under the effect of the impeller and volute coupling, in the newborn of cavitations, impeller pressure fluctuations were sinusoidal periodic variation. With the lower of NPSH and gas volume fraction increasing, impeller pressure is changing irregularly. From the beginning of the tongue, along the impeller rotating direction, pressure fluctuation gradually declines.