The invention discloses a method for exploiting coal bed methane in collapse old goafs and lower coal beds in a combined manner. The method includes surveying mines, constructing horizontal wells from the ground to intermediate rock strata of the old goafs and the lower coal beds, carrying out fracturing and extracting the coal bed methane; circularly carrying out extraction stopping-decompression extraction procedures when the unit-time coal bed methane extraction quantities are reduced and reach 20%-40% of Q<1>, and continuing to extract the coal bed methane; carrying out secondary fracturing when the extraction quantities are lower than 10% of the Q<1>, constructing drainage straight wells and arranging equipment to drain the coal bed methane; carrying out carbon dioxide gas displacement when the extraction quantities are reduced and are lower than 20% of Q<2>, and draining the coal bed methane; carrying out supercritical carbon dioxide displacement when the extraction quantities are reduced again and are lower than 20% of the Q<2>, draining the coal bed methane until the extraction quantities are reduced and reach 10% of the Q<2>, and stopping extracting the coal bed methane. The method has the advantages that coal bed methane resources in the old goafs and the adjacent coal beds can be extracted together by the aid of the method, accordingly, the cost can be saved, and the coal bed methane resources can be extracted to the greatest extent.