Abstract: The paper describes the chromatographic separation of D_2, O_2, N_2, CH_4, CO and CO_2 in He and the quantitative analysis method of D_2, O_2 and N_2 by GC. A 5A molecular sieve column and temperature program are used. The components of D_2,O_2, N_2, CH_4, CO and CO_2 in He can well be separated. He is used as carrier gas and TCD as detector, the respective sensitivities of D_2,O_2 and N_2 are measured. The sensitivity of N_2 is highest in these components and the relative sensitivities of D_2 and O_2 compared with N_2 are 0. 1 and 0.9, respectively. When the pressure of injected sample is 0.1MPa and the volume is 0. 5ml, the detection limit of D_2 is 0.004% ( n/n ). The relative standard deviations are 1% for sample larger than 3%( n/n ) component and 10% for 0. 05%～1%( n/n ) component. This method can be used to monitor these impurities in helium blanket gas of a nuclear reactor.