Fatigue crack propagation test is a very important testing way to study the metallic engineering material fatigue fracture characteristics by on-line measuring crack length under the cyclically varying loads. Crack length has been obtained through crack image capture, image processing and image analysis. In order to improve the crack measurement precision when the fatigue crack keeps growing, a new crack length real time measurement method based on the image processing and camera automatically tracking techniques was presented in this paper. This method includes the crack image processing algorithms and camera tracking and positioning movement control. The crack length measurement errors based on the camera tracking and fixed camera are respectively 0.076mm and 0.148mm.The experiment results show the proposed method has greatly improved the crack length measuring precision with the ordinary optical device.