Abstract: The steam generator is one of the crucial equipments of the sodium-cooled fast reactor. The sodium water reaction caused by the heat transfer pipe rupture generates a large amount of hydrogen and heat, which will endanger the safety of the sodium-cooled fast reactor. In this study, a numerical investigation in the sodium-water reactor test system under sodium-water reaction case of small and medium leakage was simulated based on VOF (volume of fluid) multiphase flow model. Both the three-dimensional spatial distribution and migration characteristics of hydrogen in the autoclave were obtained. The analytical results show that the migration characteristics of hydrogen in the autoclave are affected by the sodium flow rate, and the migration characteristics of hydrogen in the entire circulation loop are mainly controlled by the water leakage rate. By setting hydrogen meter with a sensitivity of 0.005 ppm, the fastest characteristic time for detecting hydrogen in different regions of the circulation loop is obtained.