Abstract: In order to reveal the rules of internal flow and energy conversion in annular pumping chamber and improve the overall hydraulic performance of reactor coolant pump, a three-dimensional numerical investigation was carried out in model pump based on Reynolds-averaged N-S equations and RNGk-ε turbulent model. The flow state, energy loss and the changes of dynamic and static pressure energy in different sections were compared and analyzed under different conditions. The results show that the hydraulic loss of the annular pumping chamber increases approximately in a line with the increase of the flow rate. For pumping chamber loss accounted for the proportion of total losses, under small flow rate and the design conditions, the increasing trend is nonlinear.When flow rate increases to 1.2Q_d, pumping chamber loss proportion changes gently. The proportion of the static pressure energy in the annular flow passage of pumping chamber is significantly higher than that of the dynamic pressure energy, and the proportion of the static pressure energy almost linearly decreases along the flow direction, and that of the dynamic pressure energy changes by contraries. The internal flow of annular pumping chamber under different conditions show significant non-axial symmetry, and the flow complexity in the area adjacent to the tongue is mainly caused by backflow and the backflow rate under off-design conditions significantly greater than that under the design condition.