The precise measurement of the focal length is directly related to the normal use of optical instruments and give full play to its technical performance. A new practical method based on the model of optical out of focus imaging and blurred image processing of quickly and automatically measuring the focal length is proposed. Based on the principle of optical imagery the mathematical model of focus measurement principle is proposed under the condition of blurred image. The proposed method adopts the least square method for fitting circles and calculates the distance between two diffuse light spots in blurred image. The lens focal length use the calculated distance is calculated. Experiments prove that the proposed method can estimate the distance between diffuse light spots with high accuracy so as to realize the measurement of automatic focus more accurately and quickly. The proposed approach takes only one blurred image, compared with traditional focus measurement method, the amount of calculation is reduced, the speed and reliability is improved, the complexity of system is reduced.