Passive Containment Cooling System (PCCS) is a crucial passive safety feature in nuclear power plants (NPPs), designed to remove decay heat in the containment and regulate pressure and temperature during accidents. This system facilitates the introduction of gaseous flow and stratification within the containment. Additionally, the thermal responses of the PCCS water tank and the state of the containment can significantly influence the heat transfer efficiency of the PCCS. In the present study, ASTEC (Accident Source Term Evaluation Code), a comprehensive severe accident analysis software, was employed to develop a model of a Pressurized Water Reactor (PWR) Generation III incorporating PCCS. The simulation results were systematically validated against experimental data, demonstrating good agreement. In the event of a Double Ended break of Direct Vessel Injection (DEDVI), the containment pressure escalated to 1.13 MPa within three days in the absence of PCCS, whereas it was maintained at 0.305 MPa with the PCCS in operation. Furthermore, the heat transfer capacity of the PCCS was observed to diminish as the temperature of the PCCS water tank increased, reaching its nadir when the water tank was at saturation point. Conversely, as the containment temperature rose, PCCS heat transfer power increased. Throughout the process, the PCCS heat transfer power can be categorized into three distinct stages: high power, low power, and saturation. The transition between these stages is primarily governed by the water temperature within the PCCS tank. Enhancing the water volume in the PCCS tank can effectively postpone the onset of the low power and saturation stages.