In order to determine the exhaust gas effect of air vent system under the action of external airflow field in the preliminary design of a ship in advance and find out the problems in the air vent exhaust gas of design scheme, the research is conducted with the methods of Computational Fluid Dynamics (CFD) and air duct test. Taken the air vent flue gas diffusion test under the action of external airflow as the research object, a three-dimensional air duct grid model with air vent boundary conditions is established to solve the viscous incompressible steady flow at different exhaust gas velocity. The distribution and diffusion of CO2 in the exhaust gas under different external airflow velocities and different air vent layouts are calculated, and the results show that the increase of incoming airflow velocity can accelerate the attenuation rate of CO2 volume fraction along the airflow direction and weaken lateral CO2 diffusion degree. The calculation accuracy is verified according to the test results, which proves that the advantages and disadvantages of air vent layout can be evaluated with the CFD technology.