Abstract: A time-accurate three-dimensional Navier-Stokes solver of the unsteady flow field in a transonic fan was carried out.The pattern and causes of flow loss caused by pressure distortion in rotor and stator were analyzed in detail.It is found that the distorted flow influences the inlet velocity angle of rotor and stator so that the attack angles vary a lot along with circumferential direction.The flow field parameters in rotor are totally different at different radial and circumferential positions.In stator,inlet total pressure distortion induces large-scale vortex,which produces backflow and increases the loss.