Abstract: The passive containment cooling system is a critical safety component on marine nuclear power platforms, responsible for dissipating heat under accident conditions. This study addresses the rapid pressure increase within the containment during the initial stages of a loss of coolant accident (LOCA), a scenario characterized by a high probability of occurrence. A significant challenge is the response delay caused by the thermal inertia of passive containment cooling system, which can take approximately 40 seconds to establish natural circulation, leaving the containment vulnerable to an immediate thermal shock. To mitigate this, the integration of phase change material (PCM) was proposed to absorb the initial high-energy impact. This transient thermal suppression effect of a eutectic E-BiInSn (Bi_31.6In_48.8Sn_19.6) alloy under varying thickness conditions was evaluated. A three-dimensional numerical model of a simplified containment was developed in ANSYS Fluent to analyze the scenario. In the simulation, high-energy steam was injected from a 60 mm break at the base of the containment for 18 seconds to replicate LOCA conditions, with a total injected mass of approximately 80 kg. The selected PCM, with a melting point of 60.2 ℃, was modeled as a layer on the inner containment wall with thicknesses of 1, 2, 3 and 4 mm. The heat transfer from the steam-air mixture to the PCM surface was governed by both convection and steam condensation. A diffusion boundary layer model, validated against COPAIN experimental data with a maximum relative error below 10%, was implemented via user-defined functions (UDF) to accurately capture the condensation process. The results show that increasing PCM thickness significantly improves temperature and pressure suppression effect. Compared to the no-PCM case with a peak pressure of 2.09 MPa, a 1 mm thick PCM layer reduces the peak pressure by 4.31%, while a 4 mm layer achieves a 12.92% reduction, lowering the peak to 1.82 MPa. The increased PCM thickness provides greater thermal capacity and latent heat of fusion, which effectively slows the temperature rise of the material itself and enhances heat absorption. For example, the total heat absorbed by the 1 mm PCM is 9.72 MJ, whereas the 4 mm PCM absorbs 24.32 MJ. Furthermore, thicker PCM promotes more effective steam condensation, the heat absorbed through condensation increases from 24.07% of the total for the 1mm layer to 40.78% for the 4 mm layer. This study demonstrates that utilizing PCM can effectively enhance the reliability of passive safety systems, providing a strong theoretical foundation for the safety design of marine nuclear power platforms.