In order to realize the real-time and automatic measurement of various indexes of digital optical bench such as focal length, top focal length, exit pupil diameter, etc., an automatic focusing algorithm suitable for digital optical bench is proposed. The algorithm uses the Brenner clarity evaluation function, the coarse focus phase uses an improved adaptive variable step size hill-climbing algorithm, and the fine focus phase uses a least squares-based curve fitting algorithm. Experimental results show that this algorithm can accurately and efficiently find the focal plane position, the focusing accuracy reaches 0. 3%, and the repeated focusing accuracy is high.