Research on the efficiency improvement of pump as turbine (PAT) is inadequate. Blade thickness is an important geometry parameter in blade design. To explore effects of blade thickness on the influence of PAT, numerical research on three different specific speeds of PATs with different blade thickness was carried out. Their performance changes with blade thickness were presented. Besides, the variations of hydraulic loss distribution with increasing blade thickness were performed. Theoretical analysis gives a reasonable explanation for the performance change. Results show thatPAT's flow versus efficiency curve ( Q-η) is lowered; flow versus head ( Q-H) curve and flow versus power ( Q-P) curve are increased with increasing blade thickness. The increase of Q-P is mainly attributed to the increase of theoretical head caused by increasing blockage of impeller inlet area. Hydraulic loss distribution analysis indicates that the total hydraulic loss within PAT is increased with increasing blade thickness. The increase of Q-H curve is a combined effect of the increase in theoretical head and the total hydraulic loss. The decrease of efficiency with increasing blade thickness indicates that the blade thickness of PAT should be as thin as possible if its strength could be met.