Abstract: An unusual aerodynamic phenomenon is encountered in designing a high through flow,high load single stage backswept fan.The fan is one of the two base schemes of the model fan ATS 2 with preswirl incoming flow and stage pressure ratio 2.33,tip radius 0.4414 m,tip velocity 490 m/s.In the flow calculation of S_2m stream surface streamline curvature,the flow near the rotor tip stagnates unavoidably and the iteration does not converge or converges irregularly.It is found out from the analysis that the phenomenon is mainly associated with the excess backswept of the rotor leading edge.