Abstract: In the operation of a pump-turbine,the process that air supply system injects the compressed air into the runner chamber to pressing the water out,is called the draft tube water depression condition. In this condition,the pneumatic and thermodynamic characteristics that the air decompress from the tank and pass through the pipe are the key problems encountered when a proper air-supply system is designed. The current design methods mainly refer to the theoretical models and empirical principles used in other research fields,like the gas venting process of the long-distance pipe system,which cannot reflect the particularities of the working environment and the structural characteristics of the air-supply system in the pump turbine. At the same time,the numerical simulation research about the air-supply system is still blank both at home and abroad,which puts forward the urgent need of some related researches. In this paper,the full-channel and unsteady numerical simulations are performed to study the draft tube water depression. Some different turbulence models,including Spalart Allmaras Model,Shear Stress Transport κ-ω Model and Scale Adaptive Simulation Model are tested in their applicability in the draft tube water depression researches. And some flow characteristics and thermodynamic changes of the key links in the air supply system are analyzed in the present work.