Objective To establish a method for quantitative analysis of tiny amounts of tritium in hydrogen below the detection limit of isotope ratio mass spectrometer. Methods Hydrogen was oxidized to produce water in a self-developed catalytic oxidation device filled with platinum hydrophobic catalyst. The effects of different experimental conditions on hydrogen conversion rate were investigated. The tritium concentration in the synthetic water was measured using a liquid scintillation counter. The tritium concentration in hydrogen was calculated according to the measurement of the synthetic water. Results When the flow rate of hydrogen was fixed, the conversion rate of hydrogen increased with the increase of the reaction temperature but increased and then decreased with the increase of the flow rate of oxygen. Hydrogen could be completely converted under optimal experimental conditions. The hydrogen samples with volumetric tritium concentrations in the range of 1 × 10⁻⁷ to 2 × 10⁻¹⁴ were converted to water at the reaction temperature of 110 ℃ and hydrogen/oxygen flow rate of 100 mL/min. The resulting water was measured using a liquid scintillation counter. The measurement accuracy was better than 2%. Conclusion This method can be used to measure hydrogen samples with tiny amounts of tritium below the detection limit of isotope ratio mass spectrometer. Our results provide data support for the calculation of the separation capacity of cryogenic distillation process.