In optical imaging system, misfocus occurs because of a nonaccuate focal length. In recent years, the improvement for misfocus problem has caught much attention in researches. This thesis is aimed to explore the misfocus improvement and analysis. Lens plays an important role in optical image system. It can focus light at one point. The distance between the focal point and the lens is called focal length. Focal length is determined by the object distance and the image distance. As light is focused farther out of the focal point, the image will blur. It is called misfocus. The general method to improve misfocus image is done by post-processing. In 1995, wave-front coding was first proposed by Dowski and Cathey. They placed the phase mask in front of the lens, and then emphasized on the processing. In recent years, more and more researches work on this field. In this thesis, a one ring phase mask is designed by modulating the phase difference in the ring to make the improvement better. Research before wave-front coding in order to achieve a closed optical transfer function for different degrees of misfocus, but we hope the phase mask can provide immediate improvement. This idea can be applied to real-time video monitoring.