In order to study the flow field distribution of vehicle air conditioning system under different working modes, based on the computational fluid dynamics method, STAR-CCM+ is used to simulate the internal flow field of the air conditioning system in the defrosting-demisting mode and the blowing mode. The rationality of the air volume distribution of each air duct and the uniformity of the velocity distribution of the windshield are analyzed. The results show that the air flow in the air-conditioning system is relatively smooth, and the velocity distribution in the front windshield A area and the driver"s field of vision of the left and right windows is not ideal when the defrosting- demisting mode is on; the velocity and air volume distribution of the two outlets in the middle air duct are not uniform under the face blowing mode, which affects the air flow uniformity. In view of the uneven distribution of velocity and air volume, the corresponding optimization scheme is proposed. After optimization, the cell relative velocity of the front windshield is significantly improved, and the cell relative velocity of the side window glass is slightly increased and the performance meets the vehicle defrosting requirements of national standard ; after optimization, the air volume distribution of the air ducts at face blowing mode is more uniform, which is conducive to the comfort of the passenger compartment. The research results of this paper can provide simulation data support and theoretical reference for the design and optimization of air duct of vehicle air conditioning system.