Abstract: Numerical simulation of 3-D unsteady turbulent flow in a counter-rotating axial fan has been performed in this paper.The distribution of pressure fluctuation,respectively,in the time domain and the frequency domain at the characteristic points on ten monitor surfaces,which include the inlet,the collector,the first electrical motor,two-stages of impellers,the second electrical motor,the diffuser and the outlet,is obtained by using the FFT method.The analytical results show that strong pressure fluctuation exists in the two-stages of impellers and their adjacent regions.The rotor-rotor interaction between the two counter-rotating impellers,the rotor-stator interaction between the two impellers and their adjacent stationary parts as well as the periodical unsteady characteristic of the flow are the main causes that result in the pressure fluctuation inside the fan.Well-designed collector and diffuser can remarkably reduce the pressure fluctuation in the inlet and outlet of the fan.The simulation results may offer help to the noise prediction and control of the fan.