Pyrolysis-GC thermal simulation experiment has been performed on the light hydrocarbon generated from the marine source rocks of central Tarim Basin, China and the monomer hydrocarbon isotope. According to the analysis of the composition and carbon isotope of the light hydrocarbon of the oil-associated gas in Tarim Basin, it is concluded that it is mainly composed of the branch alkanes. The carbon isotopic values of cyclohexane and methylcyclohexane are lower than -26‰, which are obviously lower than those from the foreland basin in Tarim Basin.