The role of clean and renewable energy such as hydropower in energy saving and emission reduction has become more and more prominent as the breakthrough of related technologies. As a kind of hydraulic machinery , energy losses in pump as turbine (PAT) always exist. In this study, entropy generation theory is introduced to analyze the distribution of hydraulic losses of PAT owing to limitations of the traditional approach based on the pressure drop method. The results of numerical simulation are in good agreement with the results of experiment on hydraulic performances. The energy losses mainly focus on guide vane , volute and runner under different flow rates, accounting for 49.73%–58.07%, 20.13%–22.45% and 16.81%–23.23% respectively. And the wall effect and velocity fluctuation are primary factors of entropy production , together accounting for more than 98%. Results of comparison between wall entropy generation rate and wall shear indicate that wall shear stress has a great contribution to the losses on the blade of guide vane and runner. Shock phenomenon, backflow and vortices are considered to be primary hydraulic factors to irreversible energy losses in PAT. This research also reveals that the asymmetric vortex rope in volute is the dominant source of entropy generation. The results of distribution of entropy generation in different components of PAT can provide certain help for optimizing corresponding structures.