Phase change materials have attracted significant attention owing to their promising applications in many aspects. However, it is seriously restricted by some drawbacks such as obvious leakage, relatively low thermal conductivity, and easily flame properties. Herein, a novel flame retardant form-stable composite phase change material (CPCM) with polyethylene glycol/epoxy resin/expanded graphite/magnesium hydroxide/zinc hydroxide (PEG/ER/EG/MH/ZH) has been successfully prepared and utilized in the battery module. The addition of MH and ZH (MH:ZH = 1:2) as flame retardant additions can not only greatly improve the flame retardant effect but also maintain the physical and mechanical properties of the polymer. Further, the EG (5%) can provide the graphitization degree of residual char which is beneficial to building a more protective barrier. This designation of CPCM can exhibit leakage-proof, high thermal conductivity (increasing 400%–500%) and prominent flammable retardant performance. Especially at 3C discharge rate, the maximum temperature is controlled below 54.2 °C and the temperature difference is maintained within 2.2 °C in the battery module, which presents a superior thermal management effect. This work suggests an efficient and feasible approach toward exploiting a multifunctional phase change material for thermal management systems for electric vehicles and energy storage fields.