Abstract: Three-dimensional flow field simulation was conducted on a two-stage transonic fan stage.The simulated fan stage performance was greatly matched with the experimental performance.Blade tip separation,blade tip leakage blocking effect and the stator corner separation were checked for filtering out the possible cause of incepting fan stage stall.Based on this study,the root cause for the stall was finally concluded as the stator corner separation,along with the mid-span boundary separation on the second stage.