Abstract: In order to study the energy loss mechanism of the axial flow pump as a hydraulic turbine, this paper takes the axial flow pump as the research object. Based on the theory of entropy production, the energy loss mechanism of each flow component of the axial flow pump under different flow conditions is quantitatively analyzed, and the energy loss mechanism of the impeller and the outlet section is specifically analyzed. The results show that under different flow conditions, the entropy production loss of the impeller and the entropy production loss of the outlet section are the main factors of the energy dissipation of the whole machine. The flow separation in the flow process of the impeller region, the low speed area and the wake flow are the main reasons for the energy loss of the impeller, and the vortex band in the downstream area of the tail cone of the outlet section is the main reason for the energy loss of the outlet section.