Abstract: In order to study the flow transient characteristic in the first impeller and diversion shell of the condensate pump in nuclear power plant, the water model of the first impeller and the diversion shell was established and simulated. The fluctuation curve of the head, the radial force vector distribution of the impeller under different working conditions and the time domain and frequency domain characteristics of the pressure pulsation in the impeller and the diversion shell under design condition were obtained. The results show that the amplitude of the head under the large flow condition is slightly higher than the amplitude of the head under the small flow condition, when the blade outlet is sweeping through the tongue, the head reaches the valley value, and when the blade outlet is gradually away from the tongue, the head increases and reaches the peak. The amplitude of the pressure pulsation from the inlet of the impeller to the outlet increases gradually. At the impeller outlet, the amplitude of the pressure pulsation decreases from the blade working surface to the blade back surface. The diversion shell pressure pulsation frequency is very close to blade frequency, and the pressure pulsation amplitude at the tongue is significantly larger than that at the other positions. The absolute value of the average radial force in design condition is less than that in other working conditions and radial force directs substantially 90° diversion of the tongue in design condition. When deviating from the design condition, the radial force produces an obvious bias.