The high-power mechanical pumps used in thermal power plants consume a considerable amount of electrical energy. Thus it is of great importance to improve the efficiency from every potential feature of such pumps. In this study, the clocking position of the pump, defined as the circumferential position of the vaned diffuser relative to the circular casing, is investigated to improve the hydraulic performance. Thereafter, the flow loss visualization method is employed to depict the loss caused by clocking positions with the help of entropy production theory based on computational fluid dynamics (CFD). The numerical results are then validated by experiments. At the design condition, the numerical maximum difference in the hydraulic efficiency is 2.61% while the head of each scheme meets the design requirement, The clocking position influences the total pressure loss coefficient more in the circular casing than other components, and the largest difference in the circular casing is 1.91%. Furthermore, the proven visualization method reveals that the clocking position introduces additional flow losses in the circular casing of the worst scheme, and even increases the loss in the vaned diffuser. This study can also provide a new perspective to investigate the optimal assembly strategy for other hydraulic machinery. (C) 2019 Elsevier Ltd. All rights reserved.