Abstract: An approximately analytical propagation formula of plane wave passing through a system with annular diaphragm was derived with the method of expanding the window function of the annular diaphragm into the product of Fourier series and complex Gaussian functions. The beam propagation through the optical system with the annular diaphragm was numerically simulated with the formula. On-axial and transverse intensity distribution is calculated and analyzed. The numerical simulation results show that the intensity distribution is related to the obstruction ratio of the annular diaphragm.