An automatic tracking collision avoidance method has following steps: taking images in front of an origin vehicle by at least two photograph units; the images delivering to a calculation assembly, to transform the images from three primary colors images to grey images; having a relative vehicle speed of the front vehicle corresponded to the original vehicle according to the grey images; and according to the grey images the calculation assembly having a relative vehicle speed that the front vehicle corresponded to the original vehicle; the calculation assembly having a safe distance according to the relative vehicle speed, if the safe distance is great than a distance that between the original vehicle and the front vehicle, an alarm module generating an alert signal.