The temperature management capacity of high-power electronic devices plays an important role in the stable output of the electronic instrument. Owing to the high latent heat thermal energy storage and nearly constant temperature of the phase change process, the phase change materials offer advantages in electronics cooling, but the low thermal conductivity of phase change material limits its heat transfer capacity. However, graphene could be used to improve the effective thermal conductivity of composite phase change materials(PCM). In this study, the realistic porous structure of composite phase change materials is reconstructed using the stochastic method. A three-dimensional(3D) multi-relaxation time(MRT) Lattice Boltzmann model(LBM) is developed to simulate the conjugate heat transfer in periodic power devices. The numerical results indicate that the addition of graphene could significantly improve the cooling capacity of the phase change materials. For the pure phase change materials, only about 0.23 of the phase change materials undergo phase change during the working process of electronic devices. The phase change process is concentrated on the heating chip side, while most of the phase change materials far from the heating chip side are not utilized. As a comparison, for the graphene composite phase change materials, about 0.68 of the phase change materials undergo phase change during the work process of electronic devices. The phase transition process mainly occurs around the heat chips and the porous graphene framework. Especially, the maximum temperature during the electronic chip operation is significantly reduced after the addition of porous graphene in the pure phase change materials. The heating temperature rise in the composite phase change material is reduced by 0.455 compared with the pure phase change material. The present study consolidates the understanding of the mechanism of the heat conduction processes in electronic cooling, contributing to the optimization of the structural design of the composite phase material. concentration, which significantly improves the local low current density at the downstream area. The cooling strategy of decreasing inlet coolant temperature is more effective than that of increasing coolant velocity, and 343.15 K inlet temperature keeps the average membrane temperature near 353.45K with excellent temperature uniformity. This work can shed light on the thermal management characteristics, design, and optimization of commercial-size PEMFCs.