When the radial polarization vector beam is focused by a high numerical aperture lens, compared with the linear polarization light and the circular polarized light cases, a smaller focused spot can be obtained by the radial polarization vector beam. An elliptic annular aperture can enhance the axial component of light in the focus area. So the focus spot size can be further reduced in one direction. The focusing characteristics of the radial polarization vector beam passing through an elliptic annular aperture are studied. Two focusing lobes are obtained by the radial polarization component of light. An elliptical focusing spot is obtained by the axial component of light and the corresponding physical explanation is given. When the inner radius increases, the focus size of the focusing spot and focusing lobes can be less than one third of the wavelength in one direction. When the inner ring radius of the elliptic annular aperture is further improved, a smaller focus size of light can’t be obtained as the decrease of light and the diffraction effect. The results of the study are important for improving one direction resolution of laser scanning microscope.