The first order radially polarized beam is with cylindrical symmetry in polarization. It is commonly thought that radial polarization leads to the best light beam focusing in terms of spot size, and the focal spot has strong longitudinal field. Using numerical calculation method, it is presented that the focal spot of radially polarized beam which is smaller than the size along polarization direction of focal spot with linear polarized beam can be obtained, if the angle between refracted light and optical axis is larger than 70°. But the size perpendicular to polarization direction of focal spot with linear polarized beam is always smaller than that of radially polarized beam. In addition, it can lead to the smaller focal spot of radially polarized beam when an annular aperture diaphragm is inserted in optical path. But this method is not suitable for the situation of linearly polarized beam along polarization direction.