A numerical simulation on the air flow in vehicle air-conditioning duct is conducted by applying computational fluid dynamics（CFD）code Fluent and its result is compared to test result within a very small difference. Besides, the result shows that the distribution of air flow is so uneven that the thermal comfort of driver is rather poor. To improve it, the air-conditioning duct system is optimized by response surface methodology which took three structural dimensions of the deflector attached to it as design variables and considered the airflow ratio of the driver side and the overall airflow as optimized purposes, and then the thermal comfort analysis of the driver was performed to the improved air-conditioning system. As a result, the airflow ratio of the driver side is improved to 51.3% from 44.9% due to the optimized air-conditioning duct, as well as that the thermal comfort of the driver is obviously improved.