Abstract: Based on the classical autocollimation method of measuring the focal length of the concave lens,the paper proposes an improved autocollimation method through which concave lens instead of image screen are moved so that the influence of focal depth on the measurement result can be eliminated. The analysis on the experimental results shows that the accuracy of measuring the focal length of the concave lens by the improved autocollimation method greatly increases，with the average decrease of relative error by 3.93% and the decrease of variance by 20.39 mm.

Key words: autocollimation method, method of moving image screen, method of moving concave lens, focal depth