Abstract: A new quantitative preconcentration method for trace hydrogen in natural gas was proposed in this paper so as to deal with the problem of isotopic composition measurement.Based on the differences of chemical and physical properties between different components in natural gas,using cryogenetic trap and vacuum techniques,helium was injected to liquefy the gas components with higher boiling point.In this way,trace hydrogen was concentrated and the concentration degree was calculated.A correspon-ding equipment was designed and produced.Experiments with the mixed gas of standard H₂ and CH₄ as well as trace hydrogen were carried out.It has been proved that,the new method is qualified to concentrate trace hydrogen.The testing error is about ±5‰.It might be a system error,but still remains confirmation.