Abstract: The vortex chamber is the main component of the vortex drop shaft spillway. To overcome the limitation of measuring cavity swirl in traditional model tests. The RNG k-ε turbulence model is used to numerically calculate the gas-liquid two-phase flow in the vortex chamber, and the hydraulic characteristics of the swirling flow in the vortex chamber are obtained. The calculation results show that the near-wall pressure in the vortex chamber is affected by the contraction section of the shaft and increases gradually with the elevation decrease. The pressure of the cross-section is symmetrically distributed along the radial direction and increases almost linearly with the radius increase. The pressure isolines on the cross-section roughly conform to the concentric circle distribution in the center of the vortex chamber, which is most dense at the air-water interface, and gradually becomes sparse toward the wall. The tangential velocity on the cross-section of the vortex chamber shows an axisymmetric distribution law, which increases with the increase in the radius, decreases with the increase in the radius at a certain distance from the wall of the vortex chamber, and then drops sharply to 0 somewhere near the wall. The axial velocity on the crosssection of the vortex chamber is the largest at the center of the section cavity, decreases first and then increases with the change in radius, and rapidly decreases to 0 at a specific position near the wall. Tangential and axial flow are the main types of three-dimensional swirling flow in the vortex chamber. As the flow moves, the flow changes from tangential flow to axial flow. The value of the near-wall swirl angle in the vortex chamber varies from 60° to 80°, and the significant value of the swirl angle in the vortex chamber indicates that the water flow in this area is mainly tangential. With the elevation decrease, the swirl angle decreases along the way, meaning that under axial gravity and with the movement of water flow, the flow in the vortex chamber changes from tangential to axial flow. The combined vortex index n reflects the internal flow characteristics of the spiral water flow in the vortex chamber. On the cross-section, the value of n increases gradually from cavity to wall in the range of-0.4～1, indicating that the water flow along the radial direction is a combined vortex movement transforming from quasiforced to quasi-free vortex. In the axial direction, the range of free vortices decreases with elevation, indicating that the cavity vortex is dominated by quasi-free vortices and transformed into quasi-forced vortices.