Abstract: Tip leakage flow in lower pressure axial flow fans with circumferential forward-skewed,backward-skewed and radical blade,were investigated numerically at the off-designation conditions and validated experimentally.The circumferential skewed effects on origination and development of tip leakage vortex were discussed according to flow rates.The results show that as flow rates decrease,tip leakage vortex origination move towards the leading edge,and circumferential skewing strengthen the mixing between tip leakage vortex and main flow;tip leakage vortex of forward-skewed blade is located most far from leading edge,vortex strength is weakest,and movement is slowest.The circumferential forward blades stabilize tip leakage flow,decrease end-wall losses at off-designation conditions and improve stable operation rang.