An energy recovery device, the double-suction centrifugal pump as the turbine has a wide application prospect in the field of large flow and high-pressure head. The impeller is one of the most important rotating flow components. Its working efficiency can pose a great influence on the energy conversion of the double-suction pump as the turbine. Meanwhile, the internal friction and unstable flow in the impeller can cause the hydraulic loss of the double-suction pump as the turbine, leading to the low efficiency and safety of the pump as the turbine operates. However, the local and wall entropy production rate can be classified as the dissipation caused by irreversible factors, according to the entropy production theory. The local entropy production rate includes the direct entropy production rate caused by non-uniform time average velocity distribution and the turbulent entropy production rate caused by non-uniform fluctuation velocity distribution. Furthermore, the location and size of the irreversible loss in the flow process can be diagnosed by the entropy production theory. In this study, a Shear Stress Transport(SST) κ-ω turbulence model was adopted to clarify the energy loss mechanism in the pump as the turbine impeller. A numerical simulation was then carried out using reasonable mesh division and an accurate boundary layer under Computational Fluid Dynamics(CFD). An external characteristic test was conducted to verify the numerical simulation strategy. Finally, a systematic analysis was made on the energy loss of each flow-through component in the pump under different flow rates, in order to determine the area of high entropy production rate in the pump as the turbine impeller. The energy loss mechanism of the impeller area was clarified to combine with the entropy production theory. The results show that the main reasons for the hydraulic loss in the whole machine were the entropy production rate of turbulent caused by the unstable flow in the impeller channel, and the wall entropy production rate caused by the internal friction in the near-wall area. The average proportions were 41% and 55%, respectively, indicating the extremely low proportion of direct entropy production rate. The total entropy production rate of each flow-through component was ranked in the descending order of the impeller, draft chamber, and volute, where the average proportions were 55%, 30%, and 15%, respectively. In the local entropy production rate of the impeller region, the uneven velocity distribution in the flow field is caused by the flow separation and vortex generated at the suction side and pressure side of the blade, the dynamic and static interference between the volute tongue and the impeller, the bending flow with strong curvature between some impeller flow channels, and the reaction of the backwater zone of the draft chamber on the internal flow state of the impeller, which is the main reason for the increase of the turbulent entropy production rate and energy loss. In addition, the entropy production rate continuously increased on the wall with the increase of flow, due to the dynamic and static interference between the volute tongue and the blade, the interaction between blade, shroud, and fluid, the sharp increase of velocity gradient near the wall, and the increase of shear force and viscous force. At the same time, the flow in the channel posed a great influence on the entropy production of the front cover wall. But, there was no influence on the rear cover wall, which was closely related to the special back-to-back impeller structure of the double suction pump. This finding can provide a strong reference for the hydraulic optimization design of the double-suction pump as the turbine. [ABSTRACT FROM AUTHOR]