The commercial recovery of methane from coal is well established in the coalbed methane (CBM) blocks at the east margin of Ordos Basin, China. CBM forms with various carbon isotopic ratios (δ¹³C_PDB) due to the carbon isotopic fractionation in biogenical or thermogenical processes. Based on the geologic evolution of coalbed reservoir and studies on the characteristics of δ¹³C_PDB values distributed spatially (e.g., horizontal CBM well location area, vertical coal burial zone, coal rank, etc.) and temporally (e.g., geologic evolution history), we explored the formation mechanism of carbon isotopic of methane. The relatively low δ¹³C_PDB values are widely distributed along the research area, indicating a trend of “lighter-heavier-lighter” from north to south. From a combination analysis of the relationship between δ¹³C_PDB and the relative effects, the essential aspects in determining CBM carbon isotope being light in the study area are: the genesis of secondary biogas in the north; water soluble effects in the active hydrodynamic areas in the middle; desorption fractionation effect promoted by tectonic evolution in the south; and the sudden warming hydrocarbon fractionation accelerated by magmatic event in particular areas (e.g., Linxian).