In view of the main problems existing in the transverse flow heat dissipation mode of cylindrical battery pack in the new energy vehicle, it was proposed that the longitudinal flow heat dissipation mode could be adopted for the battery pack heat dissipation. Based on the classical convective heat transfer theory, a theoretical study on this heat transfer mode was carried out. Through calculation and analysis, the heat dissipation method could realize the no-gap battery pack design, reduce the gap between the batteries, effectively reduce the battery pack volume, and increase the volume energy density, and meanwhile ensure the good uniform temperature performance and higher heat transfer efficiency. The results show that the longitudinal flow heat dissipation method has obvious advantages in increasing the bulk energy density of battery pack, and the heat dissipation performance can be optimized by increasing the cooling medium flow rate, which provides a theoretical basis and guidance for the heat dissipation design and optimization of cylindrical battery pack.