The air-cooled battery thermal management system (BTMS) is a safe and cost-effective system to control the operating temperature of the battery energy storage system (BESS) within a desirable range. Different from the design of the air supply flow field of most BESSs in previous studies, this study proposes a novel calculation method that combines the cooling air duct and the battery pack to enhance the heat dissipation of the battery. Using computational fluid dynamics (CFD) models, potential problems with numerical calculations of cooling air duct and battery packs alone and coupled simulations of the two are investigated. The important factors influencing the uniformity of air supply are identified, and creative measures for improvement are proposed. The results in this paper show that the uniformity of the outlet air supply does not indicate that the temperature uniformity performance of the matrix battery meets the requirements due to the variation of the sub air duct outlet pressure, and the coupling simulation of the cooling air duct and the battery pack is an essential process for BESS. With the improvements proposed in this paper, the standard deviation coefficient of velocity is reduced from 60.3% to 12.6%. Furthermore, the innovative improvement of placing the partition in the connecting duct can regulates the battery temperature between 298.58 K and 311.73 K and ensures a maximum temperature difference of only 4.22 K for a single battery. Ultimately, the power consumption of the cooling system can be reduced by 6.9%. The results of the paper provide a guide for uniform heat dissipation in BESS.