Abstract: Abstract: Due to that the design aims for structural parameters of semi-trailer tractor are multivariate and the constricted boundary condition of this designing process is complicated, the utilizations of empirical approach and the single objective optimization can't always promote the comprehensive performance of semi-trailer tractor. Based on non-dominated sorting genetic algorithm II (NSGA-II), a new optimizing method about semi-trailer tractor's structure parameters was put forward. By analyzing structural and dynamic characteristics of semi-trailer tractor's 2-DoF (degree of freedom) model, optimizing principles were established. According to the tractor operation performance including the manipulative stability, negative weight addition, limitation of driving wheel's load, load rate of engine power and adhesive characteristic, the constricted boundary conditions were designed. The optimizing objective functions were formulated, which included fuel economy, power performance and force status of tractive point. YTO-150 tractor and the matched semi-trailer were collected as the investigative and optimized object. According to the theoretical analysis and mathematical modeling of the tractor and trailer dynamic performance, the 4 objective functions were divided into 2 groups which involved contradictory relation. After the multiple mathematic conversions of objective functions and constraint functions, the complicated and coupled relationship between the optimal objects could be simplified. Using NSGA-II two times , the semi-trailer tractor's structure parameters and the gear ratios of transportation work condition were calculated. And relevant program was redacted. Parameters including the gravity of the tractor and semi-trailer, and the position of each part's barycenter were optimized. And the transmission ratio of each transportation gear was modified. The figure describing the Pareto front end of each aimed function was plotted. As compared with the primitive semi-trailer tractor, the total weight was declined by 6.86%, the position of the tractor's barycenter moved forward by 0.074 m, and the position of the trailer's barycenter moved backward by 0.14 m. As compared with the single objective optimizing result with the way of developing the CAD (computer aided design) systems of Delphi, the total weight was declined by 3.26%, the position of the tractor's barycenter moved backward by 0.022 m, and the position of the trailer's barycenter moved backward by 0.3 m. On 5 different gradient test ramps, the climbing performance experiment was performed. And in the climbing tractor and trailer, the velocity and pull force on the traction axle were measured by the GPS (global position system) device and resistance strain pull and pressure sensor. When the tractor was driven under the transportation gearⅠ, the maximum climbing degree was improved by 1.35% and 1.68%, and the average force of the tractive point declined by 1 222 and 703 N, respectively, compared with the single objective optimizing scheme and primitive semi-trailer tractor. When the tractor was driven under the transportation gearⅡ, the maximum climbing degree was improved by 1.38% and 0.57%, and the average force of the tractive point declined by 2 792 and 2 125 N, respectively, compared with the single objective optimizing scheme and primitive semi-trailer tractor. The fuel economy of the tractor trailer systems was simulated by the simulator, which was developed upon the dynamic joint between AVL CRUISE and MATLAB. When the simulation adopted the marked working condition based on EUDC (extra urban driving cycle), the fuel consuming rate of the multi-objective optimized semi-trailer tractor declined by 12.9% and 15.8%, respectively, compared with the single objective optimizing scheme and primitive semi-trailer tractor. To sum up, this optimized method reaches the requirement of the objective functions, and provides the theoretical and technologic foundation for improving tractive vehicle systems.