Abstract: Based on the turbine trip test results of Monju fast reactor, the whole development process of thermal stratification after the turbine trip in upper plenum, using holes with sharp and round edge on the inner barrel, was simulated. The stratification characteristics including thermal stratification forming, interface rising speed, temperature gradient and flow ratio of flow hole to the annulus gap were investigated. The results show that the numerical simulations agree well with the experimental results and the thermal-hydraulic phenomenon in fast reactor can be preferably forecasted by numerical simulation under some conditions. The experience of building a numerical simulation method to forecast complicated transient state case is accumulated.