Flow of the passage mid span section in a low speed axial compressor is simulated using two dimensional Navier Stokes equations.It is found that the calculated results are in good agreement with the experimental data.The characteristics of blade force frequency chart are investigated by using the Fourier series method.Results show that for different axial gaps between the rotor and stator,fluctuation of the unsteady blade force is mostly determined by the first rank Fourier component.Simplifying the fluctuation as sine function,it can be derived that as long as the number ratio of the disturbing to the disturbed blade row is not integer,the total unsteady blade force fluctuation from the disturbed blade row will approach to zero.