Optical superresolution, focal shift and high focal depth in optical imaging system have been the goal of many researchers and have widely application in the optical microscopy, optical alignment and optical coherent tomography systems. A tunable pupil filter for modulation of axial focal shift and extended focal depth is proposed. It consists of one half-wave plate located between two identical and parallel quarter wave-plates, the half-wave plate is made up of two zones that can rotate with respect to each other, and the pupil function is deduced. The research result shows that with the designed pupil filter the axial focal shift can be controlled by rotating the half-wave plate. So the designed tunable pupil filter can be utilized in automatic focusing applications that are similar to optical read-write drives for which is necessary to track the focal plane dymamically. Furthermore, when the phase difference is π, the transverse superresolution and axially extended focal depth can be realized at the same time. And it can be used in optical confocal scan imaging system.