The phase change material (PCM) can be used in thermal energy storage (TES) with large latent heat. However, due to the heat accumulation in the upper region, the solid-liquid phase change process will be slowed down. In this paper, in order to enhance the heat transfer of nanoparticle-enhanced PCM (NEPCM) in TES, a separate plate is applied to weaken the heat accumulation. The phase change multiple-relaxation-times lattice Boltzmann (MRT-LB) model is employed to solve the numerical problem and the effects of separate plate location, nano particle volume fraction and Rayleigh number are investigated. The results show that for all the Rayleigh numbers and volume fractions, the NEPCM of the case that separate plate is located in the middle of cavity melts fastest. Furthermore, the separate plate can reduce the temperature standard deviation. However, when the location of separate plate is less than 0.3, the melting process is slowed down, resulted from the heat accumulation near the separate plate.