Abstract: A small steel containment bench was built to study the circulation and thermal stratification in passive containment by using steam as the working medium during the loss of coolant accident (LOCA) or main steam line break (MSLB) accident. The results indicate that the stratification always exists in passive containment at different jet flow rates. However, it is a partial stratification instead of stratifying in whole space. Based on the comparison of main containment calculation codes and experimental results, a new calculation method of containment was proposed. According to the characteristics of heat transfer and flow, the space in containment was partitioned to different regions and then models were established respectively.