The passive containment cooling system has become integral to the safety design of modern nuclear power plants. In the Chinese small modular reactor ACP100, the passive containment air-cooling system (PAS) is implemented to remove decay heat through air convection in the annular gap between the steel containment and the concrete containment. This study focuses on analyzing the thermal–hydraulic safety performance of PAS after a severe accident. Using a severe accident analysis code, models of ACP100’s primary loop and PAS were established. Simulations were conducted to evaluate PAS response after a Loss-of-Coolant Accident (LOCA) induced by the breakage of direct vessel injection line. Simulation results indicate that, after a LOCA in the ACP100 reactor, the primary circuit coolant discharges rapidly, leading to a sharp increase in containment temperature and pressure, with the peak pressure reaching approximately 2.8 atm. Subsequently, due to the cooling effects of PAS and the structural components, the pressure gradually decreases and stabilizes within the defined safety limits. These findings confirm that PAS effectively manages containment pressure and temperature, meeting the requirements of nuclear power plant safety standards. This study demonstrates the reliability and safety of PAS under severe accident conditions, highlighting its valuable engineering potential.