Abstract: Base on the vectorial Rayleigh-Sommerfeid diffraction integrals,analytical expressions of plane waves with unity amplitude propagating through a thin lens followed by one small circular aperture are derived,for the non-paraxial and paraxial approach.Using these expressions,numerical calculations of the transverse distribution of optical intensity and the distribution on the axis have been done.It is found that the non-paraxial approach should be applied if the aperture dimension is comparable with the wavelength,and paraxial approach should be applied if the aperture dimension is larger than the wavelength.