Abstract: A method for the analysis of high mature natural gas by Gas Chromatography with on-line preconcentration technique is introduced. High mature natural gas with greater than 0.95 of dry coefficient flowed through a self-developed enrichment back-blow device to trap the trace light hydrocarbons. After the back blow of un-liquefied components, the trapped trace light hydrocarbons were gasified and transferred into Gas Chromatography with carrier gas by heating the cold trapped components, which significantly expanded the analysis range of light hydrocarbons according to the results. The light hydrocarbons that were less than C₁₀ were successfully analyzed with this method. The results of three experiments of natural gas in Puguang 7 showed a good reproducibility in indices of methylcyclohexane, n-heptane, iso-heptane and Mango K₁ which were 0.22, 0.23, 0.02 and 0.00, respectively, lower than the national standard, 0.82, 0.75, 0.11 and 0.04. Meanwhile the solvent effect caused by methane has been dramatically reduced. The parameters obtained with this on-line analysis method can be effectively applied to the studies of origin and maturity of natural gas.