The direction of fast-speed and heavy-duty highway transportation imposes demanding requirements for smooth driving, stabile operation and safety of heavy-duty vehicles. The position of the loading center of gravity is the key factor affecting heavy-duty vehicles' driving. For a certain type of heavy-duty vehicle, its equivalent support position and center of gravity is analyzed and determined through the complete calculation method, and its centroid safety area is studied. Then the calculation method for ultimate load analysis and the software realization of the safety area of the loading centroid are introduced. Through analyses of the centroid safety area and the ultimate load of a certain type of heavy-duty vehicle, we offer a new method and idea for the follow-up study of vehicle kinematics