In order to find out the effect mechanism of the cavitation on the hump characteristic of pump-turbines, three dimensional numerical simulations with and without cavitation were carried out using the SST (shear stress transport) k-omega turbulence model and ZGB (Zwart-Gerber-Belamri) cavitation model. Numerical results agreed well with the performance characteristic and flow visualization results. Under cavitation conditions, the simulations with cavitation obtained a more accuracy prediction on the hump characteristic. The relationship between the performance characteristics and the cavitation was analyzed in details. A cavitation condition (cavitation coefficient is 0.14), under which the cavitation just occurs, was selected to investigate the hump characteristic. The occurrence position of the cavitation and the variation law with the cavitation coefficient were obtained. The hump characteristic is related to the cavitation on suction surfaces close to the leading edges of the blades at the runner inlet. The action mechanism of the cavitation on the change of the hump characteristic was revealed. The sudden increase of the cavitation in the hump region leads to the decrease in the Euler head and the increase in the hydraulic loss, resulting in the drop of the head. Detailed analyses reveal that the occurrence of the cavitation will block the runner passages, which reduces the working ability of the runner and induces large size vortices in the adjacent passages. (c) 2020 Elsevier Ltd. All rights reserved.