In this paper, the cooling performance of the parallel air-cooled Battery Thermal Management System (BTMS) is improved through designing the spacing distribution among the battery cells. Computational Fluid Dynamics (CFD) method is employed to calculate the flow field and the temperature field of the BTMS. Then an optimization strategy combined with the CFD method is used to adjust the cell spacings, with the target of minimizing the maximum cell temperature difference. Typical cases are used to test the effectiveness of the proposed optimization process for cell spacing optimization. The results indicate that the maximum temperature of the battery pack is reduced by approximately 3.0 K and the maximum cell temperature difference is reduced by more than 60% after optimizing the cell spacing distribution. By choosing appropriate initial cell spacing distribution and step length of the cell spacing adjustments, optimized results can be obtained in short time without sacrificing the performance of the solution. Furthermore, the cell spacing optimization does not increase the total power consumption, and the optimized BTMS achieves good cooling performance for various inlet airflow rates. It is suggested that the presented optimization process is an effective method to design the cell spacing distribution and to improve the cooling performance of the BTMS. (C) 2018 Elsevier Ltd. All rights reserved.