Composite phase change material (CPCM) as passive battery thermal management system (BTMS) still confronted many challenges such as easy leakage, high rigid and low thermal conductivity. In this study, a multifunction flexible CPCM with high anti-leakage and thermal conductivity performances has been proposed which is utilized the polymerizing and crosslinking reaction with polyethylene glycol and hexamethylene diisocyanate to address the leakage of CPCM in essence. The results revealed that the CPCM exhibited excellent antileakage and elastic performance. Especially, the thermal conductivity of CPCM is obviously improved by 2.8 times owing to the synergistic effect of 3 wt% aluminum nitride and 2 wt% carbon nanotubes. Besides, the maximum temperature of the battery module with multifunction flexible CPCM can be controlled below 45 °C even at 1.5C discharge rate, and the corresponding temperature difference is maintained within 4.3 °C. With these outstanding properties, the multifunctional flexible PEAC can provide an efficient solution to improve the thermal safety of battery packs in applications, which will significantly promote the development of EVs.