Abstract: The air core vortex is a common hydraulic problem in front of the intakes placed on the hydraulic works. A model experiment method is a usual way to study the hydraulic characteristics of the vortex in front of the intakes. At present, the previous studies of the vortex hydraulic characteristics are mainly focused on the intakes of the spillway tunnels or water intakes of the hydropower stations, which have the unchanged structures and deep-water submergence. In contrast to these model tests, there is a few studies of the hydraulic characteristics of the vortex in front of the partially opened radial gate. In order to study the hydraulic characteristics of the vortex in front of the partially opened radial gate and influences of viscous force and surface tension on this vortex, the air core vortex formed in front of the partially opened radial gate of a flood releasing sluice was taken as a research object in this study. A theoretical analysis method and two comparative experiments of different scale models were carried out to study the air core vortex in front of the partially opened radial gate. The results show that the vortex formed in front of the partially opened radial gate is a result of transverse and vertical contraction of the progressive flow. And the turbulent fluctuation of the progressive flow in front of the radial gate is strong, so the vortex frequency in front of the gate is high and the inspiration duration of the vortex is short. Further, the more the radial gate opened, the stronger the intensity of the air core vortex is. If the experimental models are operated according to the Froude number similarity criterion, the influences of the viscous force and surface tension on the modeling vortex can be ignored when the Reynolds and Weber numbers are larger than a certain critical value. The research results of this paper can provide a reference and basis for future researches and engineering designs.