Abstract: The mechanism of multi-composition coalbed gas fractionation during the process of solution can be interpreted by Henry Law.This mechanism is a guide for the investigation of coalbed gas accumulation.Henry's constants of methane and carbon dioxide can be calculated by the experience formula,and the fugacity can be calculated by fugacity gene formula,based on Henry Law,the soluble mole fractions of carbon dioxide and methane in groundwater with burial depth increasing were calculated.The result indicates that methane solubility increased with burial depth increasing,and carbon dioxide solubility increased at beginning and then decreased with burial depth increasing(critical depth of 800 m);but the solubility rate(c_CO2/c_CH4) decreased with burial depth increasing.This means that carbon dioxide soluble fractionation is stronger at shallow depth,and it becomes weaker with burial depth increasing.This mechanism can ascertain the coalbed gas accumulation,and can provide a theoretical foundation for ascertaining the optimal condition of injection carbon dioxide to enhance methane production.