Abstract:The baffle-drop shaft is widely used in deep tunnel drainage system due to its fine applicability and high energy dissipation. To fully reveal the high speed water-air two-phase flow characteristics in baffle-drop shaft during discharge process, a scale of 1∶25 physical model test and the numerical simulation based on the Realizable k-ε model and volume of fluid (VOF) method were carried out. The distribution law of water and air in the baffle-drop shaft was studied under different flow conditions. The formation mechanism of cavity area in wet side and the variation process of air pressure in dry side were expounded. And the distribution law of shaft wall pressure and hydrodynamic loadings on different baffles were analyzed. The results showed that the formation of “cavity area” in the baffle-drop shaft was caused by the internal air compressed to the limit, and another reason came down to the negative pressure caused by the entrainment of the water flow in wet side. At large inflow, the distribution law of flow velocity in drop shaft was low velocity on central baffles and high velocity on upper and lower baffles. But when the inflow was small, the flow velocity decreased gradually along the direction of the shaft depth. The air pressure varied from -0.96 to 2.46 kPa under the top-sealed condition, and the air pressure on the dry side did not change over time when the relative vent diameter of shaft cover is greater than 0.105. The wall pressure of the drop shaft increased with the decrease of the shaft height, and the maximum wall pressure of the upper, central and lower baffles of the shaft were 42.25 kPa, 21.5 kPa and 16.75 kPa, respectively. The research results provided a scientific basis for the design theory and safe running of the baffle-drop shaft of deep tunnel drainage system.