Tritium is a key nuclide for radiation protection monitoring in tritium related sites. It is of great significance to effectively monitor the concentration of gaseous tritium. According to the requirements in Part 5 of GB/T7165.5-2008, the reference tritium gas consistent in composition with the object to be tested should be used in the tritium activity monitoring and calibration. To solve this problem, we need to study and establish the preparation method and certification method of reference tritium gas to ensure the effectiveness of the source of tritium gas activity measurement. Tritium reference gas is prepared based on dilution method, the tritium activity concentration is measured based on length compensation method, and the measurement uncertainty is analyzed. The development of tritium gas standard source fills the gap in this field in China and provides an important support for the traceability of tritium monitoring instrument.