Abstract: The development of a new method to predict the unstalled characteristics and the onset of flow instability is described for an axial compressor operating in a circumferentially distorted downflow.This method adopts the“semi-actuator disk”to replace the blade rows of the compressor and the downstream distorting component,and assumes the flow fields outside the disks to be two-dimensional,ideal,compressible and unsteady.The effects of total pressure distortion coefficient and spacing between the compressor and the distorting component on the performance of a compressor have been investigated in detail by means of this method.It is found that the losses of both pressure rise and axial flow coefficient at instability increase linearly with the distortion coefficient,this conclusion is similar to that for the inlet flow distortion.And the spacing has crucial effect on the losses of both pressure rise and axial flow coefficient too.