A numerical method is presented to predict the circumferential distortion of the flow in the volute by an iterative updating numerical procedure for both the impeller and volute. By such method, flow field of a centrifugal blower volute is calculated. The results show: in the designed working condition, the circumferential distortion of the flow in the volute inlet is negligible; while drifting off the designed working condition inlet flow distortion is very strong and makes the mass flow deviate far from the uniform distribution.