Abstract: To study the stall mechanism of a transonic axial compressor, numerical simulations were conducted on the NASA Rotor 35 combined with throttle valve model. To further research the variation behavior of the flow field during stall process, an analysis was carried out on the flow field in the tip region using the method of proper orthogonal decomposition (POD). Results show that the stall originates in the tip region of blade and develops into a stall cell rotating coaxially with the rotor in the same direction but with lower speed. By comparing the flow field characteristics at different time, the leakage flow is found to exist throughout the process of stall development, which could be taken as a basis for judgment of flow field worsening. Meanwhile, the structure of large-scale unsteady flow could be identified in the flow field during stall process.