The imaging of the cross wire target on the CCD is generally affected by the ambient light and the optical path. The cross wire image generated by the focal length of different positions will be blurred and out of focus, and the extraction of the center point of the cross wire target will be seriously affected. The traditional Sobel algorithm ignores the edge information and is easily affected by noise. Therefore, a two-stage fast search auto focusing method with variable-step size is proposed, which improves the image sharpness evaluation function of Sobel operator and combines the coarse and fine focusing. The method first evaluates the image sharpness by using the spatial domain of the image, and then selects the two-stage focusing mode according to the optical design. The first step is coarse adjustment. When the focus position is quickly found, fine adjustment is performed until the focus position is found. Experimental results show that compared with other algorithms, the auto focusing failure rate is 2%, the focusing time is 1242 ms, and the focusing stroke is -25-30 mm. The combination of the image sharpness evaluation function and the two-stage focusing mode through this algorithm has high accuracy and good real-time performance.