Abstract: The integrated severe accident analysis MAAP5 program is applied to establish the containment model of the third generation passive nuclear power plant. Selecting the double end break superimposed PRHR failure and IRWST gravity injection failure of the the primary pipe as the initiating events, and the accident process, hydrogen source term, the effectiveness of post-inerting hydrogen mitigation measures and the post-inerting injection location are studied. The results show that the cumulative hydrogen production of zirconium alloy and water reaction is 266 kg, and the peak release rate is 0.55 kg/s during the whole accident; After the accident, the post-inerting mitigation measures of that CO₂ injection into hydrogen can effectively reduce the risk of hydrogen combustion inside the containment, and the effect of inerting CO₂ injection into the source compartment of 1# SG is more obvious than that of injecting the same amount of CO₂ into the upper part of the containment. In order to mitigate the risk of hydrogen in the containment by post-inerting, we must consider the containment air extraction system to prevent overpressure from threatening the integrity of the containment.