Abstract: In order to meet the regulation requirements of the new power system, pump turbine need to operate within a wider range. Low load operation of the pump turbine leads to a decrease in the unit' performance, affects the safe and stable operation. Therefore, the prototype pump turbine was selected as the research object of this paper, the internal flow characteristics of the pump turbine under different operating loads were studied using numerical simulation methods. The relationship between operating loads, energy loss, and flow structure was analyzed using entropy production theory. The results showed that the entropy production loss is mainly pulsating entropy production. When the load decreases, the relative flow angle decreases, and low-speed zones are formed at the blade inlet due to flow separation. At 50% load condition, the low-speed zone alternately appears inside the runner. The vortex structures appear along the circumferential direction at the blade inlet, blocking the water flow into the channel, resulting in a significant increase in entropy production loss and proportion inside the runner. The tangential velocity at the runner outlet is closely related to the formation of vortex rope in the draft tube. As the load decreases, the tangential velocity gradually increases, and the circumferential motion trend increases after flowing into the draft tube. The vortex rope develops from cylindrical to spiral. In the low load zone at depth, vortex rope develops into double spiral vortex rope. The research results provide theoretical support for the safe and stable operation of pumped storage power stations.