Abstract: In order to explore the abnormal hydraulic excitation phenomenon of the pump-turbine in the low-load operation condition, based on the PIMPLE algorithm in the OpenFOAM software, combined with the SAS-SST turbulence model, a three-dimensional numerical simulation for the pump-turbine in different low-load operation conditions was carried out.After comparing the test data, it is proved that the algorithm has certain stability and feasibility in the simulation of low load conditions.From the simulation results, it can be found that when the unit is operating away from the rated condition, low-frequency pulsation is prone to occur in the vaneless area, and there are backflow vortex structures on the suction surface of the blade and the outlet edge.As the flow rate increases, the internal flow state of the runner is improved, and the eccentric spiral vortex at the straight cone section of the draft tube under the condition of small opening gradually evolves into a concentric cylindrical vortex, and the movement process of the vortex will affect the flow state and the uniformity of pressure distribution at the inlet of the draft tube, which is the main factor inducing the draft tube vortex.