Abstract: The sodium-cooled fast reactor is one of the important reactor types of the fourth-generation advanced nuclear energy system. The safety of sodium-cooled fast reactors is improved by introducing a new passive decay heat removal system. The China Experimental Fast Reactor (CEFR) adopts the direct reactor auxiliary cooling system (DRACS) method. The passive decay heat removal system is directly installed in the primary loop, which can further improve the decay heat removal capability of the sodium-cooled fast reactor under accident conditions. However, due to the complex structure of the pool fast reactor and the special arrangement of the passive decay heat removal system, the innovative design of DRACS also brings about problems such as “multipath” of natural circulation. In the current numerical simulation research on the natural circulation of pool fast reactors, it is difficult for the system program to accurately predict the complex natural circulation paths in the pool, and it is difficult to accurately simulate the three-dimensional thermal and hydraulic phenomena in the pool, and the three-dimensional CFD computational modeling and meshing are more difficult. It is high and requires large computing resources. Combining the advantages and disadvantages of one-dimensional programs and three-dimensional CFD, this study developed a “1-D system+3-D CFD” coupled method for calculation and analysis of the natural circulation characteristics of the fast reactor passive decay heat removal system. The innovative method was verified using the DRACS natural circulation mode of Japan’s large-scale sodium loop experimental bench (PLANDTL). The relative error of key position parameters under steady-state operating conditions is less than 3%. The change trend of key position parameters under transient operating conditions is in good agreement with the experimental values, the relative error is less than 10%. Which verifies the applicability and accuracy of the coupling method. Using this coupling method, calculation and analysis of the CEFR natural circulation and decay heat removal characteristics were carried out, the natural circulation flow path in the pool is identified, and the temperature stratification and “inter-wrapper flow” phenomenon in the pool are revealed, which are three-dimensional transient characteristics of the natural circulation of large sodium-cooled fast reactors. This coupled method can provide important numerical methods for the three-dimensional transient characteristics analysis of natural circulation in large sodium-cooled fast reactors