To address the argon leakage that occurred in the protection vessel of a sodium-cooled fast reactor during long-term operation, this paper constructs a detection method based on operation records, pressure change analysis, and system isolation tests. Using the ideal gas equation of state, pressure and temperature data were converted into volumetric leakage rates, showing that the average daily leakage of the protection vessel is approximately 300–430 L·d-1. By isolating the external argon distribution and overpressure protection system, combined with interface leak detection, it was confirmed that the main leakage source was concentrated at the sodium leakage detection point interface. After completing tightening and sealing repairs, the leakage rate decreased to approximately 40 L·d-1 after 48 hours, improving the ability to maintain the inert gas environment.