Due to the limited height of the engine room of some fishing boats, there are inevitably structural stiffeners in the ventilation pipeline. The type and height of the stiffeners, as well as the number and position of the ventilation grilles, will affect the flow field in the air duct, resulting in differences in ventilation effects. Aiming at the ventilation system of the engine room of a certain type of ocean fishing vessel, the engine room port and port air ducts were simulated based on the CFD numerical method, and the relationship between the airflow field, stiffeners, and ventilation grilles was studied, and the motion characteristics of airflow in obstacles or developing flows were considered. The results show that different types of reinforcing rib structures have different effects on the flow field of the air duct, and the air duct with a smooth inner surface has the smoothest flow state; the increase of the height of the structure in the air duct will cause the disturbance of the flow state, reduce the wind speed, and increase the unbalance of the wind speed of each ventilation grille; it can indicate the direction for the optimization of the design and optimization of the ventilation system of the engine room of the fishing boat.