The invention discloses an image-based vehicle anti-collision early warning method. An image sensor is utilized for collecting the images of a rod ahead; after a vehicle is detected successfully and tracked, according to the vehicle width variation in interframe images and combining with the influences of the relative vehicle speed and relative acceleration, the time to collision (TTC) of a target vehicle and the vehicle is calculated, the dangerous level is analyzed, and an alarm is given to a driver, and meanwhile, the relative driving state of the target vehicle and the current vehicle also can be determined. The method provided by the invention is applied to the technical field of vehicle active safety, can effective prevent rear-end collision with the vehicle ahead, also is not influenced by rugged topography, and has high safety performance.