The continuous braking of the brake drum will cause the brake thermal decay when the heavy truck is driving down the long slope in the mountain areas. This reduces the braking performance and the braking safety. The engine braking and the retarder braking both consume the kinetic energy of the truck and can assist the truck driving in the mountain areas. This research proposes a combined braking strategy of the heavy truck in the mountain areas based on the mountain GIS information and the former recorded information of the slope gradient. The accelerator pedal is released in the middle of the uphill to assist the engine braking and access the shift between the kinetic energy and the potential energy. Different braking methods are allocated properly according to the slope length and gradient in the downhill. The vehicle dynamic model is established to analyze the speed variation and the truck’s braking characteristics on the long slope. The characteristics of the engine braking, the brake drum braking and the retarder braking are considered based on the mountain GIS information and the former recorded slope gradient. Then the combined braking energy with the length and the gradient variation of the slope are analyzed. All of these guarantee the braking safety. The results show that the combined braking strategy increases the fuel economy by 7%-12.3% on the uphill. The truck speed is controlled and the usage frequency of the brake drum is reduced on the downhill, which ensures the braking safety on the long slope.