Abstract: Transporting in coal seams with porous media,methane can result in significant isotope fractionation,which cannot be fully explained by the existing models and mechanisms. Therefore,the dynamics model for fractionation of carbon isotopes of methane is developed based on the theories such as competitive adsorption among different molecules,the difference of diffusion capacity and the desorption diffusion causes of isotope fractionation. The results are shown as follows:the carbon isotopes of methane become increasingly heavier during the process of desorption. The instantaneous isotopes become quickly heavier at an exponential speed during the latter period of desorption. The variety of accumulative isotopes is similar to that of accumulative amount of degassing gas. During the process of diffusion,the diffusion coefficient ratio (D1/D2) has a significant effect on isotope fractionation. The content of adsorption gas in coal core accounts for more than 90%,and the variety of isotope fractionation caused by adsorption/ desorption is close to that of isotope fractionation caused by coupling process,which means that the model better predicts the experimental results of carbon isotope fractionation for desorption of methane in coal core. The important cause of isotope fractionation is the process of desorption and diffusion for methane in pore,where the isotope fractionation caused by free gas adsorption gas is of stage wise. According to the match of parameters between model and coal core,the content of adsorption gas and free gas can be obtained to evaluate the content of methane gas in coal seams. When applied to the process of desorption and diffusion for gas in coal seams,its basic parameters can be determined through on site and laboratory measurements. The model is applied to the process of gas drainage,its parameters are determined by combining with field test and laboratory test,and then the relationship between the carbon isotope value of gas extracted from coal seam and gas drainage situation is established. The stage of coal seam gas drainage can be assessed,and the real time residual gas content of coalbed around drainage borehole can be obtained,which can provide a theoretical basis for the potential application of evaluation effect of coal seam gas drainage.