In this study, a load rejection process of a pump-turbine was simulated using the large eddy simulation method with and without considering the hydraulic acoustic effect. In the simulation, the hydraulic acoustic effect was considered via a revised density function which is called weak water compressibility model. Analysis showed that numerical results considering water compressibility and without the consideration both agree well with the experimental data except some local regions. Compared with the experiments, the simulation deviations of the rotational speed were less than 5%, and the local maximum deviations of the fluctuating pressure were no more than 30% during the whole load rejection process. However, the simulation considering the water compressibility has a better behavior than that without the consideration in the predictions of pressure fluctuation. And the performance characteristics with the consideration are systematically lower than those without the consideration. In some operating conditions, the performance characteristics with the consideration are even 30%-50% lower than those obtained without the consideration. Additionally, it is noted that the fluctuation of the axial hydraulic thrust on the runner with consideration is more severe than that without the consideration. Hence, the water compressibility has an obvious influence on the simulation of the load rejection of pump-turbines. (C) 2020 Elsevier Ltd. All rights reserved.