Abstract: In order to improve the hydraulic performance of the nuclear main pump and achieve optimal matching with shielded motor, based on the scaling model conversion algorithm, selecting RNG k-ε turbulence model and SIMPLEC algorithm, unsteady numerical predictions and external characteristic experiments were conducted. The results show that on the flow conditions of 0.4Q_d-0.7Q_d, the head-flow curves are relatively flat; on the rated condition, the predicted value of head is 5% higher than the rated one; the maximum head of the impeller is at the 0.4Q_d operating point, the maximum value of hydraulic efficiency is located at the 0.9Q_d operating point, the simulation value of hydraulic efficiency of the impeller is 5% higher than the experimental one. On small flow rate conditions, the hydraulic loss of guide vane yields the normal distribution law taking the 0.4Q_d condition as the central axis, the maximum value of guide vane hydraulic loss is at the 0.4Q_d operating point. On large flow rate conditions, the minimum hydraulic loss value of the guide vane is at the 1.1Q_d operating point. The hydraulic loss of water pressure chamber is in accordance with sinusoidal distribution law; the peak is located at the vicinity of 0.4Q_d operating point and the trough is near the 0.9Q_d operating point.