Pump as turbine (PAT) is a type of energy recovery equipment. However, the mechanism of performance degradation when it deviates from the design working condition is inadequately reported. Hence, the steady performance of a centrifugal PAT with a design specific speed (n(sd) = 3.65n(d) root Q(d)/H-d(0.75)) of 90 was simulated by ANSYS-CFX 18.0. The shear stress transport (SST) k-omega turbulence model was used to close the Reynolds-averaged Navier-Stokes (RANS) equations and the simulation results of performance were verified by comparing the experimental. To explore the energy loss mechanism of the PAT under different flow conditions, the energy loss of each part of PAT was calculated by the enstrophy dissipation method, which not only accurately calculate the energy loss but also diagnose where and how hydraulic loss occurs. Compared with the traditional pressure drop assessment method, the enstrophy dissipation method has higher prediction accuracy for the rotating domain, almost the same prediction accuracy in the static domain. The energy loss of the PAT mainly occurs in the cavity, volute and impeller. Hydraulic loss is dominated by fluctuating enstrophy dissipation power and wall enstrophy dissipation power. Adverse flow phenomena such as backflow and shock flow could evidently increase the volume and wall enstrophy dissipation rates. Therefore, the proposed method can be used intuitively to provide a refer-ence for PAT performance optimization and actual operation regulation. (C) 2020 Elsevier Ltd. All rights reserved.