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In the machine vision fatigue crack propagation test, in order to observe the crack macroscopically and locate the crack tip accurately, the zoom lens was needed to enlarge and narrow the image. For the problem that the camera needed to be re-focusing after zooming, an auto-focusing method was proposed. After establishing the image acquisition system, the crack image was collected to computer, and then median filter was used to pre-processing the image to remove the noise. The crack area was selected as the focusing window and evaluated sharpness of the area by Laplace operator method, and then an adaptive step size exhaustive method was used to search the focus. The controller based on ARM7 was designed to control the motion of camera. The research results show that for all situations the proposed method can realize the accurate auto-focusing of the camera in the fatigue crack propagation test, and it built the foundation to accurately measure the fatigue crack growth parameters.