Abstract Pump manufactures normally do not provide performance curves of their pumps working as turbines, especially performance curves of trimmed impellers. Impeller trimming is necessary to adapt to a certain operating condition. Therefore, establishing the correlation of PAT’s performance curve and impeller trims is essential. In this paper, a laboratory model of a pump as turbine (PAT) open test rig was set up. Experimental research was carried out on a single stage centrifugal PAT, performance curves of original impeller, impeller after once and twice trims were acquired. Numerical simulation and analysis were performed on the PAT. The accuracy of CFD was validated through comparison between numerical and experimental data. As impeller diameter is cut down, its geometrical parameters of impeller diameter, blade wrap angle, impeller inlet width and blade inlet angle are changed. So, research on effects of the above four parameters to the performance influence of PAT were carried out using CFD. Through analysis of the predicted performance curves, the reasons for the variation of performance curves after impeller trimming were explored.