The internal flow structure and the energy loss in the first stage impeller side chamber of a molten salt pump for solar thermal power generation were investigated numerically. The flow field in the model pump was simulated based on the RANS equation using the standard k-ε turbulence model. The results indicate that the rotating speed of core flow in the front impeller side chamber is higher than the tangential velocity at the maximal radius of the front shroud. However, the core flow in the rear impeller side chamber gives an opposite trend. Meanwhile, the radial velocity at the boundary-layer separation point on the front impeller side chamber stationary wall decreases initially and then increases with the radius while it only decreases in the rear impeller side chamber. For the energy loss, the percentage of the disk friction loss to total energy consumption reduces as the flow rate increases, while the absolute value of disk friction loss on the front shroud keeps almost constant and the loss on the rear shroud decreases with the increasing flow rate.