This study aims to solve this problem and understand the formation mechanism of the vortex attached to the side pump sump. Thus, numerical simulations were conducted using the calculation software, and the flow characteristics of the side pump sump were studied based on the vorticity field. The flow field data of the physical model of the pumping station forebay and pump sump were obtained by particle image velocimetry. The distribution of vorticity in the different flow layers and vertical axes at the study location was obtained using Tecplot postprocessing software and corroborated with the results of the numerical simulations. This basis was used to analyse the mechanism of the formation of the adherent vortex. Results show that the inflow distribution is uneven and the water flow resistance and vortex strength are imbalanced due to the asymmetry of the vorticity of the inflow of the side pump sump. This causes the attachment of the vortex to the wall, which causes the generation of the suction vortex, vibration of the unit, and instability of the overcurrent, endangering the safety of the project.