The invention relates to a cargo vehicle security state early warning method based on braking distance, in particular to a determination method of the early warning threshold of the on-road vehicle. Aiming at the condition that the on-road vehicle is easy to have some potential dangers, the method of the invention uses the braking distance as the starting point to reflect the secure state of the vehicle. The real-time monitoring method of the on-road state of the vehicle comprises the following specific steps: extracting various factors of the vehicle such as the brake parameters, the currentstate factor and the outer road environment factor to perform information integration, using a braking distance model to calculate the required braking distance d1 of the current vehicle, comparing with a safe braking distance d0, calculating the degree of risk to determine the current danger condition of the vehicle, and using the range of the degree of risk as the early warning threshold of thevehicle to perform graded early warning. The invention has high instantaneity, precision and accuracy, does not require complicated calculation and design and provides a scientific and effective method for the determination of the danger condition threshold of the vehicle.