Abstract: The research on safety margin is always the key and difficult problem in reactor safety design and safety analysis. The demonstration fast reactor (CFR600) is a pool type sodium cooled fast reactor. In the accident of primary loop loss of flow accidents such as primary loop main pipe break, the core flow decreases rapidly and the cladding temperature will peak. Under the superposition of extreme conservative assumptions, the calculation results show that the core will cause damage. The conservative method for accident analysis greatly limits the economy and flexibility of power plant. The best estimation analysis method can provide information closer to the reactor behavior, distinguish most related safety issues, and obtain the safety margin more accurately. The core of the best estimation method is the best estimation model and uncertainty analysis technology. Based on the analysis of the important phenomena of the main heat transfer system, the thermal stratification effect model established based on the plume/jet theory, solves the problem that it is difficult to analyze the thermal stratification effect on the thermal hydraulic characteristics of the system in the zero dimension or one dimension model of the system program. The pump cost down model based on the time parameters was proposed, which solves the problem that it is difficult to obtain the input of the inertia of the pump and the resistance characteristics of the pipeline system in the traditional model. The numerical method based on the characteristic line method to solve the flow network of the primary system was improved, which is suitable for solving the severe transient accidents such as main pipe rupture. Intermediate heat exchanger (IHX) thermal hydraulic model was developed, which can simulate the backflow phenomenon in the primary side of IHX. Based on the Leibniz theory, the sliding grid model of once through steam generator was established, which can effectively solve the interface jump phenomenon of the fixed grid model in the transient process. After the main pipe breaks of the primary loop, a large number of coolants quickly discharge from the two broken ports, fault loop has obvious backflow phenomenon, and the heat transfer characteristics of the two loops form obvious asymmetry phenomenon. The upper limit of peaking cladding temperature (PCT) 95%/95% is 851 6 ℃, which is reduced by 91 8 ℃ compared with the conservative method, and lower than cladding damage acceptance criteria.