Stimulated emission depletion (STED) microscopy can break the diffraction limit, which enables scientists to discern cell details smaller than 200 nm. By radially polarized beam through the high numerical aperture (NA) lens, the resolution of STED microscopy systems can be improved. The distribution of radial intensity and axial intensity is calculated, furthermore, the effect of NA on the distribution of radial intensity and axial intensity is analyzed. The research shows that the size of axial focal spot is smaller than that of the radial focal spot, the spot size decreases when the NA increases, the full width at half maximum (FWHM) of the depletion beam decreases as well, and the intensity distribution becomes more concentrated. The increase of NA can improve the resolution of STED microscopy systems.