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In order to study the relationship between the cavitation flow and energy conversion, based on the continuity equation, Renault N-S equation and RNG k-ε turbulence model, the whole flow field cavitation numerical simulation on unclear main pump model under design condition is carried out. Through the variation of pressure and velocity on the streamlines, combined with the basic equation of the pump, the dynamic and hydrostatic head of the nuclear main pump under different cavitation conditions are studied. The results show that the energy of fluid of nuclear main pump is provided by posterior segment of impeller, and the energy of the fluid decreases gradually from the shroud to the hub. Cavitation interferes with the flow of liquid in the impeller, which leads to the relative velocity increases and the pressure decreases in cavitation region, and the work capacity of blades is almost zero in bubble areas. At the same time, in non-cavitation region along with streamlines the dynamic head increases and the hydrostatic head decreases with the development of cavitation, and the decrease of the hydrostatic head is greater than the increase of the dynamic head, which results in the decrease of pump head and efficiency. In addition, in the cavitation region, with the development of cavitation, the sudden change of the dynamic and hydrostatic head increases, which increase the flow loss in the impeller and results in the decrease of pump head and efficiency moreover.