The focusing of optical measurement system is one of the important steps in the process of obtaining high-quality images. When the EO measurement system during target tracking and imaging is unable to timely focus, the obtained fuzzy images can adversely affect the attitude, tracking, recognition and radiation characteristics measurement of the target. Aiming at this problem, a new phase-image hybrid passive focusing structure was designed on the basis of the summary of current focus detection methods in ground-based field.In addition, a focusing detection prototype was made . Through the compared experiments on the prototype between the proposed method and the traditional image focusing detection experiment, the phase focusing accuracy of the system is better than 0.08 mm, and the compound focusing accuracy is better than 0.02 mm, which can effectively improve the focusing efficiency of the system.