The importance of renewable energy will increase rapidly due to the global regulation of carbon dioxide emissions to cope with global warming. However, renewable energy has a high output variation because of the fluctuating characteristics of the energy source. Therefore, pumped-storage power stations require high flexibility and reliability in operation under off-design conditions, especially in the pump mode. When a pump-turbine operates under various part load conditions in pump mode, highly dynamic phenomenon such as stationary vortex and rotating stall occur. The performance characteristics in pump mode are vital for the safe and effective operation. A number of studies have been conducted to investigate the flow characteristics in a turbine or pump mode under different GVOs through numerical simulations. However, the studies about the influence of the position of the interface and the steady and unsteady interface condition on the pump characteristics of pump-turbines have been not completely clear. In this paper, the three-dimensional steady and unsteady Reynolds-averaged Navier–Stokes equations were solved for a detailed analysis of the influence of interface conditions with various guide vane opening conditions in pump mode. To ensure the reliability of the numerical analysis, the numerical results were validated in comparison with the experimental data. •The effect of interface conditions with various guide vane openings was studied.•The numerical results were validated in comparison to the experimental data.•The dynamic flow phenomenon and hysteresis characteristics were investigated.•The location and magnitude of vortex under the hump region were observed.•The unsteady pressure characteristics were analyzed with internal flow field.