The invention provides a safety early warning method during running of vehicles on the freeway and a system. The method comprises steps of detecting the running speed of a basic vehicle and the running speed of the back vehicle via a radar speed detection device installed over a lane, wherein the basic vehicle and the back vehicle are on the same lane; detecting vehicle distance between the basic vehicle and the back vehicle via a high-definition video installed over the lane, obtaining image information and determining license plate information according to the high-definition camera; determining the safe vehicle distance threshold between the basic vehicle and the back vehicle; judging whether the vehicle distance is smaller than the safe vehicle distance threshold or not; and if the vehicle distance is smaller than the safe vehicle distance threshold, displaying first early warning information and the license plate information by use of a variable information plate installed over the lane. According to the invention, it can be achieved that following distance of vehicles can be detected, whether one vehicle is very close to another vehicle can be judged, and distance too-close early warnings can be given to follower drivers; traffic accident rate is reduced; and traffic safety level is increased.