As a way of exploitation and utilization of ocean energy, the waterjet pump is used in a wide range of high-speed marine vessels over 30 knot. This paper aims to investigate the mechanism of the energy loss and pressure fluctuations caused by the non-uniform inflow for a waterjet pump. Unsteady internal flows inside the waterjet pump are simulated using the Reynolds-averaged Navier-Stokes equations with the SST k-omega turbulence model. The predicted pump head and efficiency are in reasonable accordance with the experimental data. The inflow non-uniformity would decrease the hydraulic head, efficiency and increase the axial force fluctuations in the impeller, causing large pulsations in the unsteady energy performance. Based on analyses of the energy loss, the turbulent kinetic energy production and the diffusion of the Reynolds stress are major sources of the energy loss in the waterjet pump. The non-uniform inflow induces a dramatic energy loss in the intake duct and diffuser with an apparent flow separation observed near the trailing edge of the diffuser blade. Due to the inflow non-uniformity, the pressure fluctuates violently at the impeller rotating frequency (f(n)) in the intake duct, impeller and near the diffuser inlet, but a dominant frequency of 2f(n), is generated by the unsteady flow separation near the diffuser outlet. (C) 2020 Elsevier Ltd. All rights reserved.