Abstract: It is necessary to inject air into the runner passage to reduce the starting power, when pump-turbine operates under the starting process in pump mode or synchronous condenser mode. In order to prevent the compressed air in the runner passage from leaking into the casing, it is necessary to form a certain thickness of water-ring in the vaneless space for sealing to reduce the air consumption. In order to investigate the water-ring characteristics of pump-turbine in synchronous condenser mode by different guide vane openings, the ANSYS CFX software was employed to conduct the unsteady state analysis with two-phase flow. The pump-turbine model was adopted to investigate the internal flow characteristics on the water-ring with three guide vane openings（full closed, 1% GVO and 2% GVO）. The result shows that the thickness of the water-ring increases with decrease of guide vane opening, which results in increasing the resistance of the runner and the consumption of the shaft power. At the same time, the dominate frequency of pressure fluctuation in the vaneless space is the blade pass frequency and harmonics, which represents the phenomenon of rotor-stator interaction obviously. The research results can provide a reference for controlling water-ring of pump-turbine in the process of synchronous condenser mode and pump start-up mode.