The depth of field of standard optical imaging system can be extended by introducing a pure phase pupil mask. A pupil mask which contains three zones with “0、π” phase is presented. By modulating the pupil of the imaging system, the Modulation Transfer Function (MTF) of the system has showed defocus-independent. Therefore, the loss of system’s imaging information is avoided meanwhile the depth of the field is enlarged. Finally, the large depth of field system is simulated by Zemax as well as the traditional optical system. From the experimental results, we can draw the conclusion that the binary phase mask has successfully extended the depth of field of imaging system with a higher resolution.