In order to solve the problem of low search efficiency in autofocus sy stem,this paper proposes an autofocus algorithm based on clear domain.The algo rithm innovatively proposes the concept of clear region,and the problem of find ing the clearest image in auto focus system is transformed into the problem of f inding clear area.At the same time,in order to solve the problem of secondary peak interference in the actual focusing process,a high pass filtering method is proposed to combat the secondary peak interference.In this paper,two groups of experiments are designed to judge the feasibility of the proposed algorithm by comparing the performance of the search efficiency before and after the applic ation of the algorithm in this paper.Experiment 1verifies the feasibility of high pass filtering for eliminating the secondary peak interference,and Experime nt 2verifies the feasibility of the clear domain autofocus algorithm for improving the search efficiency.The results of Experiment 1show that the high pass filter has certain suppression effect on the secondary peak interference without distortion.The results of Experiment 2show that the high pass filter can improve the search efficiency by 8%－18% when the clear region is applied to the image after high pass filtering.