TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: When the working gas replacement uses an operation mode of replacement with pressure, the methane is replaced and absorbed by carbon dioxide under an absorption pressure by (A) entering the raw material coal-bed gas into an absorption tower at the normal temperature in which the absorption pressure is 0.3-1 MPa, enriching the strong absorption component methane in the raw material gas in an absorbent bed layer and flowing a weak absorption component nitrogen out from the tower top, stopping the gas leading when methane concentration distribution in the absorption tower moves forward to a position close to the tower top but still not penetrating through the top end of the absorption tower, and finishing the absorption step; (B) after the absorption step is finished, continuously entering the carbon dioxide in the absorption tower, replacing the methane absorbed on the absorbent by the carbon dioxide, further enriching the methane gas and flowing the gas product out from the tower top, stopping the carbon dioxide leading-in when carbon dioxide concentration distribution moves forward to the top end of the absorption tower, and finishing the replacement step; (C) after the replacement step is finished, enabling the absorption tower to regenerate by blowing or vacuuming with nitrogen as side product; (D) leading the side product into the absorption tower after regeneration to pressurize the pressure in the tower to an absorption pressure; and (E) regenerating the absorption tower using a vacuuming method, feeding the pumped carbon dioxide into a buffering tank and recycling the pumped carbon dioxide after being pressurized, or regenerating the absorption tower using a nitrogen blowing method, separating nitrogen gas and carbon dioxide by the absorption of absorbent or a general separation method and recycling the reclaimed carbon dioxide. When the working gas replacement uses the normal pressure displacement operation mode, the methane is replaced and absorbed by carbon dioxide under the normal pressure by (A) entering the raw material coal-bed gas into the absorption tower at the normal temperature, enriching the strong absorption component methane in the raw material gas in the absorbent bed layer and flowing the weak absorption component nitrogen out from the tower top, stopping the gas leading when the methane concentration distribution in the absorption tower moves forward to a position close to the tower top but still not penetrating through the top end of the absorption tower, and finishing the absorption step; (B) after the absorption step is finished, connecting the absorption tower with another regenerated absorption tower, and using the nitrogen in the tower for pressurizing the absorption tower for ending up the regeneration step; (C) consequent decompression: after finishing the pressure equalizing step, consequently reducing the pressure in the absorption tower to the normal pressure, where a front edge of the methane concentration distribution shall not penetrate the top end of the absorption tower at this time; (D) after the pressure in the absorption tower is reduced to atmospheric pressure, switching an air inlet passage to replacement beginning step of carbon dioxide gas, continuously entering the carbon dioxide into the absorption tower to replace the absorbed methane, further enriching the methane and flowing the methane out from the tower top to form the gas product, stopping the leading-in of carbon dioxide when a front edge of the concentration distribution of the carbon dioxide is close to the top end of the absorption tower, and finishing the replacement step; (E) after the replacement step is finished, enabling the absorption tower to regenerate by side product blowing or vacuuming; (F) after the regeneration step, connecting the absorption tower with another absorption tower which finishes the absorption tower to perform pressure equalizing operation to finish the first pressurization of the regenerated absorption tower; (G) after ending up the pressure equalizing, secondarily pressurizing the absorption tower by the side product with pressure generated by the absorption tower in the absorption step to increase the pressure in the absorption tower to the absorption pressure; and (H) regenerating the absorption tower using the vacuuming method, feeding the pumped carbon dioxide into the buffering tank and recycling the carbon dioxide after being pressurized, or regenerating the absorption tower using the nitrogen blowing method, separating nitrogen gas and carbon dioxide by absorption of the absorbent or general separation method, and recycling the reclaimed carbon dioxide. Preferred Parameter: The absorption pressure of the raw material coal-bed gas entering the absorption tower at normal temperature is 0.3-1 MPa.