Abstract: This paper analyzed inner flow of an axial flow fan in helicopter at different work conditions by the ways of theoretical analysis,numerical simulation and experiment.The results show that,theoretical hypothesis of relative velocity in the impeller is applicable for rated condition but not for surge condition.When the helicopter flies at high altitude and the fan works at low flow rate condition,backflow will appear firstly in blade tip region and the secondary flow loss increases;increment of helicopter flight altitude also causes separation of boundary layer and widening of jet-wake area,thus the fan efficiency further decreases;stalling will first appear in tip clearance area in the event of gathering of low speed flow.So it is important that research on off-design performance should be performed to optimize main aerodynamic parameters influential to fan loss,and also to improve the stability of altitude performance when designing axial flow fan for lubrication system of helicopter.