In order to elucidate the relationship between structure evolution and carbon isotope of alkane gas in coal pyrolysis process, we selected low rank coal to carry out pyrolysis experiments in autoclave closed system. The carbon isotopic composition of alkane gas was analyzed, and the evolution of coal structure was studied using Fourier transform infrared spectroscopy (FTIR). The relationship model between carbon isotope composition of alkane gas and structure evolution was established. Moreover, the reasons for changes in carbon isotope of alkane gas were revealed. The results showed that light carbon isotopes fractionation of lipid chains cause heavy carbon isotopes to be retained in long chains when Ro, max＜1.3%. The materials within the loop, containing heavy carbon isotopes, are not fractionated out heavily when 1.3%＜Ro, max＜2.0%, and carbon isotope of alkane gas still depends on the further fractionation from lipid chains. Namely, the changes in carbon isotope of alkane gas are mainly caused by lipid chain orientation fracture when Ro, max＜2.0%. Since then (Ro, max＞2.0%), due to the intensification of aromatic condensation and disintegration, the heavy carbon in the loop can be released into the alkane gas, and its carbon isotopic composition rapidly becomes heavier. There is a synchronization phenomenon between carbon isotope composition of methane and ethane and structure evolution of lipid chain, and the values of δ13CCH4 and δ13CC2H6 can be used as the sensitivity index of structure evolution of lipid chain. In addition, it was pointed out that Ro, max=1.3% and 2.0% are important nodes in the relationship between structure evolution of coal and carbon isotopic composition of alkane gas in pyrolysis process. The research results have theoretical significance in exploring the coupling relationship between hydrocarbon generation of coal and structure evolution and mechanism of secondary hydrocarbon generation.