Based on vectorial diffraction theory, the intensity distribution of radially polarized beam (RPB) at focal point is calculated. In the focusing process of the RPB, the size of focus spot is mainly decided by the light at the verge of field. According to this characteristic, a continuous phase filter function with the tangent of the field angle as independent variable is designed. A group of optimized phase filter parameters is acquired by non linear rules algorithms, targeting on a smaller focal spot and constrained by several Strehl ratios. The results show that the focused RPB through the proposed a phase filter, a shaper focus could be obtained with the same Strehl ratio.