The passive containment cooling system (PCCS) plays a significant role in ensuring the safety of nuclear power plants, and applying energy storage technology to PCCS can promote the absorption of heat inside the containment in the early stages of accidents. In this paper, paraffine Wax is used as an energy storage material, and the melting process of Paraffine was simulated by using STAR CCM + . To verify the simulation's accuracy, we compared experimental data on the phase transition process of paraffin wax in a vertical annular container. The results showed that the temperature change trend at the different local positions was consistent, indicating the accuracy of the simulation process. On this basis, the temperature distribution, liquid phase ratio changes, and flow characteristics of the paraffin melting process during the melting process were analyzed. It can be seen that although the Paraffin near the heat transfer tube melts first, the solid–liquid melting interface does not change synchronously radially from top to bottom. The melting rate increases as it goes up, and there is an apparent natural convection phenomenon in the molten liquid phase zone, further accelerating the melting of the Paraffin in the upper region. In order to accelerate the melting phase transition process of Paraffin, it is necessary to consider enhanced heat transfer structures inside the paraffine wax to promote rapid absorption of external heat.