Abstract: System analysis code is one of the most important tools for reactor safety analysis, and can also be used to simulate the system transient test. The natural circulation test performed during the French Phenix end-of-life test, which provides a very reliable benchmark for sodium-cooled fast reactor (SFR) system transient behavior research, is very valuable for SFR. In order to analyze this test, the Phenix was modeled with the self-developed system analysis code FR-Sdaso, and the natural circulation test was calculated by the code. The calculation results were compared with the test data. The results show that the overall response of the reactor is predicted reasonably well by FR-Sdaso and FR-Sdaso can be used for this kind of SFR safety analysis.