Abstract: The source of tritium in atmosphere was analyzed, and the composition and the principle of the system for measuring tritium concentration were described. The dependences of temperature of catalysis furnace, gas flowing rate on the catalysis efficiency and dependences of temperature of cooling trap on the tritium recovery ratio were studied. The tritium recovery ratio of the electrolyte enrichment device was calibrated, and the tritium concentration in atmosphere around a nuclear facility was measured. The results show that the concentration variation of low-level tritium in the outer atmosphere can be highly sensitively measured within a special range and the low limit of tritium detection is 0.1 Bq/m³ at the catalysis temperature of 350 ℃ and the catalysis efficiency of 98% as well as the average tritium recovery ratio of 54% of the electrolyte enrichment device.