Multi-objective optimization design method based on kriging model and NSGA-II algorithm is adopted to investigate the influence of air-inlet grille to aerodynamic performance and underhood cooling performance of vehicle. Data mining technologies including total variation analysis and self-organizing map analysis are used to uncover the influence mechanisms of design variables on objective functions. Three parameters of the grille including the grille inclination angle, the number and width of grille bar are chosen as design variables. The optimization objectives are the drag coefficient, the lift coefficient and the air mass flow-rate into the underhood of vehicle. Data mining results shows out the effect degrees of variables to objectives, the relationship between variables and objectives, and the relationship between objectives. At last, aerodynamic performance and underhood cooling performance of the optimized models and initial model are compared.