This study investigated the effects of underhood structure parameters (two types of air ducts, two types of inlet grilles and the opening angle of inlet grilles) on the cooling characteristic of the rear-engine bus, and then the optimum design scheme of underhood was obtained. The air-side resistance load of the cooling system which based on fan performance was selected as optimization objective. Based on porous media model and standard k-ε model, the simulations were created. After that, 1D/3D coupling simulation was built to utilize the advantages of 1D simulation’s fast convergence speed and 3D simulation’s extensive research range. Besides, the use of 1D/3D coupling simulation can efficiently avoid the errors of simulation results which arise from the non-uniform airflow on the cooling module. Results show that the airflow rate of the rectangular air duct increased by 7 to 11percent compared with that of underhood without air duct, the air resistance load of underhood with rectangular air duct was less than that of underhood without air duct. The airflow rate of the vertical bar-shaped grille was higher than that of the horizontal bar-shaped grille, and it was highest at 45°opening angle. According to the results, the air duct and grille were chosen as the critical design variables. The optimal design scheme of underhood was obtained by investigating the combined effects of air ducts and inlet grilles on the cooling performance of the engine. When the underhood structure consists of the rectangular air duct and the 45°opening-angle of the vertical bar-shaped grille, the airflow rate comes to the maximum. In addition, the cooling air resistance load becomes lower, the lowering speed of air resistance is getting quicker as the vehicle speed accelerates and it tends to be steady at 90km/h. The new scheme is effective at improving the cooling capacity.