Using the CFD simulation method based on the volume of fluid (VOF) two-phase flow model, the characteristics of a multi-channel flat heat pipe developed for a lithium-ion battery was studied. Compared with the circular channel structure, the rectangular channel structure has a higher mass transfer rate of condensation/evaporation and a higher average velocity in the channels. The equivalent thermal conductivity of 6 square-channel flat heat pipe is 30.5% larger than that of the original structure with 6 circular channels. The heat pipe performance increases with the increase of channel number, but the increasing rate slows down gradually. With the fixed geometric parameters and the power input of the evaporation section, the performance of the multi-channel flat heat pipe is determined by the combined effect of condensation in the condensation section and the return efficiency of condensate liquid flow. As a result, the increase of the length of the condensation section does not substantially change the temperature difference between the two ends of the heat pipe, and the equivalent thermal conductivity does not reflect the effect of the length of condensation section on the performance of the heat pipe.