Carbon isotope fractionation of coalbed methane (CBM) as a kind of adsorbed gas occurs during diffusion transport of coalbed methane (CBM) in coal reservoir. In order to investigate the effect of reservoir physical proporties on the isotope fractionation, a canister desorption experiment of coal was carried out with the samples of different maturity, collected from Jincheng of Shanxi and Changji of Xinjiang, and the δ13C1 values of all the desorbed gas were recorded carefully. The results show that for the consolidated coal sample, the measured δ13C1 values of desorbed gas increase with time, and change rate is characterized with fast in the beginning and slow at the late stage. For the unconsolidated coal sample, the measured δ13C1 values of desorbed gas decrease firstly and then increase, and this is because the drilling operation and matrix shrinkage which damaged the original structure of coal during desorption influenced the normal isotope fractionation. Micropore abundance increases with increasing coal maturity, which imposes much restriction on the gas flow and results in distinct isotopic fractionation. Meanwhile, due to the effect of fracturing on the isotope fractionation, the δ13C1 values of methane collected in well head change irregularly in trial exploitation.