Abstract: Passive containment cooling system is one of the important safety systems in AP1000. The heat produced in the containment can be transferred to the atmosphere based on natural circulation both in the containment and in the air flow channel outside the containment. The flow circulation and thermal stratification in the large volume of the containment have essential effect on the flow and heat transfer. In this paper, one-dimensional thermal-hydraulic model was established based on stratification theory for the natural circulations in and outside the steel containment. The calculation efficiency was improved and the temperature distribution was obtained. By comparing with the result of the three-dimensional model, the rationality of the one-dimensional model was verified. The pressure and constituent distributions in the containment were also calculated.