Abstract: It’s necessary to establish a high-precision analytical method of gas chromatography for achieving the rapid detection of trace components in fuel cycle system of fusion reactors. H₂ and D₂ in high purity helium were separated with a handmade cryogenic capillary column of Al₂O₃, and measured by discharge ionization detector. The results show that the detect limit of this method is less than 1×10^-8, retention time is less than 180 s, the relative standard deviation of area response of H₂ and D₂ is not more than 1.0%, and the resolution of H₂ and D₂ is above 1.0. This method is sensitive and fast, and can be applied to fuelling system and isotope separation system of fusion reactors for tritium safety analysis and accurate measurement.