Abstract: In order to pre-determine the ventilation effectiveness of the closed cabin ventilation system and to identify the potential problems in airflow organization or temperature distribution in the preliminary design，the steady three-dimensional turbulent flow inside the ventilation system is simulated via CFD. Then，several substantial improvements on the system are made，which includes the addition of inlet and outlet vents in the front wall of the engine rooms and changing the airflow direction on both sides. The test result is highly consistent with the estimation from empirical formulas and shows that the proposed optimi? zation scheme significantly improves the airflow organization and ventilation effectiveness.