Based on vectorial diffraction theory, optical imaging system with continuous phase optical pupil filter for radially polarized beam is investigated. A continuous phase filter with cosine function is proposed and a sharper focus of radially polarized beam is obtained. This kind of filter is optimized with Matlab optimization toolbox and the aim of optimization is to reduce the superresolution parameter G with the Strehl ratio S as a constraint. Some examples of optimization are given. The optimization results show that there is a significant improvement on sharper focus by using this continuous phase filter with cosine function when the numerical aperture is 0.8. The numerical results about the continuous phase filter with cosine function will also help the manufacturing of actual devices.