In order to study how the leakage flow affect the vibration characteristics of rotors in centrifugal pump and calculate the rotordynamic forces from the unsymmetrical pressures acting on the shroud, the front cavity leakage flow was modeled, and an analysis of numerical simulation of rotordynamic forces arising from the front cavity leakage flow was presented in a centrifugal pump. Firstly, numerical simulation of whole flow field was conducted on a centrifugal pump. Secondly, the simulated results was used as the initial conditions of numerical simulation of the front cavity, and the velocity and pressure distributions of the interior flow field of the front cavity passage were obtained and analyzed. Both normal and tangential rotordynamic forces for various whirl frequency ratios were calculated. Last, rotordynamic forces and all six rotordynamic coefficients were calculated via a second order curve fit, and the results showed that numerical simulation could accurately calculate the rotordynamic forces arising from the front cavity leakage flow.