Abstract: In this paper,the mechanism of raising the recovery ratio of coalbed methane by gas injection was discussed.It was considered that injected gases not only enhanced reservoir energy.but also completed with coalbed methane for adsorption or replaced and expelled coalbed methane.As a result,the recovery ratio of coalbed methane was raised.By the isothermal adsorption and desorption experiments of coal samples to pure CH₄ gas as well as CH₄-N₂ and CH₄-CO₂ two binary gases,their Langmuir equations were obtained respectively,and their maximum theoretical recovery ratio was compared and analysed.The results revealed that the maximum theoretical recovery ratio of binary gases in coal beds was much higher than that of pure coalbed methane under the condition of the same gas content.