The invention belongs to the field of video detecting, and provides a video-based safety precaution method for vehicle merging from a highway ramp. The method includes arranging two cameras in the same directions as two ways on highway entrances to detect moving vehicles, marking vehicle trajectories, acquiring actual moving distance through calculation according to the vehicle trajectories in a continuous tracking frame, acquiring actual speed of the vehicles, acquiring time difference of vehicle entering the merging areas between the two ways through the speeds of vehicles on the two ways, and judging possibility of vehicle collisions through the time difference. Compared with the prior art, the video-based safety precaution method is not limited by environments, capable of performing real-time and reliable precaution for all vehicles in the video areas, easy to realize, high in accuracy, applicable to real-time traffic precaution for highway ramps, and wide in application prospect.