Downhill mountain roads are the accident prone sections because of their complexity and variety. Drivers rely more on driving experience and it is very easy to cause traffic accidents due to the negligence or the judgment failure. Traditional active safety systems, such as ABS, having subjecting to the driver's visual feedback, can’t fully guarantee the downhill driving safety in complex terrain environments.To enhance the safety of vehicles in the downhill, this study combines the characteristics of vehicle dynamics and the geographic information. Thus, through which the drivers could obtain the safety speed specified for his/her vehicle in the given downhill terrains and operate in advance to reduce traffic accidents due to driver's judgment failure and avoid the brake overheating and enhance the safety of vehicles in the downhill.Firstly, the characteristics of the driving conditions under the change of various slope lengths and gradients are studied based on models of both engine braking and wheel braking. The real-time information and characteristics of the vehicle on the mountainous area, such as the weight, the speed, the gear and the engine RPM, the vehicle braking torque characteristics on the mountainous area are analyzed. Meanwhile, the electronic map and GPS positioning are utilized to calculate the road gradient information. Finally, geographic information, road traffic speed limits and the brake efficiency will be combined to obtain a suggested safe speed at the top of the downhill roads and give as safety speed pre-warning to the driver.Simulation results and case studies the pre-warning scheme works well for downhill roads, especially for the length from 1 km to 1.5 km and the slope gradient ranged between 3% and 6%. It suggests that the proposed scheme is more effective for improvement of the safety driving in the hilly slope area, where the slope is not too steep and the slope length is not too long and can be widely used in such areas to protect the safety of downhill driving.