The invention relates to an on-board intelligent alarming method and a device, wherein, the method combines a longitudinal anti-collision pre-warning method based on the single-eye vision and an accident automatic help-calling method, realizes preventing the rear-end accidents, strives for helping time, reduces injuries and deaths, effectively prevents the collision accidents and personnel deaths caused by the collision, and improves the running safety of the automobile; the device mainly comprises three parts: an on-board terminal, a monitoring center and a user end; the method and the device use the machine vision technique to differentiate the moving vehicles in the front, have high precision and wide visible range, can effectively probe the obstacles, reduce the error reporting possibility and prevent the collision accidents. The method and the device can improve the safety of the vehicle, thus having extremely large application value and prospect.