Abstract: Ionization chamber is an important means for on-line measurement of tritium containing gas. Its response is fast but it is affected by many factors. With the development of fusion reactor and other tritium-related systems, on-line tritium activity concentration measurement is more accurate. There are gases with different hydrogen isotope forms (such as H2O, HTO and HT) in the helium extracted from the tritium carrier band of the envelope of fusion reactor. In order to study their influence on the tritium measurement in the ionization chamber, a tritium measurement experiment system was established in this study, which mainly included gas allocation unit, gas path circulation unit, ionization chamber unit, chromatographic analysis unit and water vapor drying unit. The system can accurately simulate the cycle state of tritium extraction in the cladding of fusion reactor, and can realize the functions of gas preparation, gas concentration measurement and tritium activity measurement. In this paper, the compound effect theory and He+H2 system theory in ionization chamber tritium measurement, combined with ionization chamber, chromatography, dew point meter and other measuring means, were used to analyze the influence of different hydrogen isotope gas forms containing H2O, HTO, H2 in He on tritium measurement. In the experiment, wet high-purity helium, wet tritium-containing helium and dry tritium-containing helium were prepared respectively and slowly filled into the circulation system. In this way, the influence of water vapor on the measurement of ionization chamber and chromatography was studied, and the ion recombination effect during the measurement of ionization chamber under high concentration of tritium was analyzed. The influence of hydrogen on the enhancement effect of tritium measurement in the ionization chamber was verified by measuring the tritium activity concentration  in helium carrier gas with different hydrogen concentration. The results show that the detection efficiency of hydrogen isotope gas by ionization chamber and chromatography is lower in water vapor environment than in dry gas environment. In order to ensure the accuracy of measurement, water vapor should be separated or removed during measurement. When the concentration of H2O, HTO and HT in the carrier gas increases, the recombination effect in the measurement of the ionization chamber is enhanced, resulting in the measurement results of the ionization chamber being lower than the actual value. The problem can be solved by increasing the working voltage of the ionization chamber or modifying the calibration recombination coefficient. When hydrogen exists in the environment of helium carrier gas, the tritium activity concentration measurement result will increase due to the influence of ionization enhancement effect, but when the hydrogen content exceeds 2 000 ppm, the ionization enhancement effect will approach the saturation state.