Abstract: Aiming at the problem of long time-consumption for automatic focus, by analyzing the change rule of image definition during the focusing process, the effective interval was determined within the focusing range. The drive rate was adjusted according to the slope of the definition change, so that a variable speed control method for focusing mechanism limited interval was improved and realized. Compared with the conventional control methods, the proposed method has the advantages of short stroke, fast speed and high precision. The experimental results show that the shortest focus time of the proposed method is less than 1.5 s, which can be used as a common automatic focus control method for similar optical lenses.