Abstract: In order to study the flow characteristics of the nuclear main pump hydraulic components deeply, based on the continuity equation, Reynolds time-averaged N-S equation and RNG k-ε turbulence model, the three-dimensional incompressible turbulent flow of nuclear model main pump with a contracted ratio of 0.4 was simulated under the three typical operating conditions (0.8Q_d, 1.0Q_d, 1.2Q_d). The pressure distribution and flow field distribution of the nuclear model main pump under different operating conditions were compared and analyzed. The flow characteristics of the impeller, guide vane and casing were described and the causes of these characteristics were discussed. At the same time, the numerical calculation of the head and efficiency values were compared with the test results of closed experimental equipment. The model pump's external characteristic curve was plotted at 0.8Q_d~1.2Q_d. The results show that the numerical analysis method, based on ANSYS CFX, can effectively predict the hydraulic performance of the nuclear main pump and obtain the internal flow details. Under the optimum design condition, the hydraulic performance of the nuclear main pump is optimal and the flow state remains uniform and stable. In comparison to the other non-design conditions, the flow characteristics of the nuclear main pump can be mastered in a comprehensive manner, which provides a reference for the design and optimization of the hydraulic components of the nuclear main pump.