To investigate the efficiency and mechanism of treating landfill leachate by two-stage membrane biofilm reactor (MBfR) in cooperation with desulfurization and decarbonization of landfill gas, we sequentially examined the removal efficiency of organic matter and nitrogen pollutants , as well as the conversion characteristics of H2S, CO2, and CH4 in landfill gas during the operation of pre-short-range nitrification MBIR and post-MBfR. After 280 days of operation, experimental results demonstrated that the reaction system successfully achieved high-efficiency desulfurization and decarburization for advanced treatment of leachate along with upgrading landfill gas. By controlling low dissolved oxygen levels in the pre-reactor, nitrosation rates exceeding 85% were achieved while enabling partial nitrification coupled DAMO-Anammox process. The average removal rates for COD, NH4+, and TN reached 95%, 99%, and 99% respectively, thereby achieving efficient carbon and nitrogen reduction for landfill leachate. Following landfll gas purifcation processes  CO2 content was reduced to less than 0.2%, and H2S content decreased to approximately 5%, while CH4 content increased to around 80%. This significant upgrade in landfill gas can be attributed to highly active denitrifying anaerobic methanogenic archaea species within the electron transfer system which enhance CO2-reducing CH4 production through direct electron transfer between species. Furthermore, this study highlights how effcient landfill gas upgrading is facilitated by enrichment of electroactive Methanothrix and Thiobacillus organisms on one hand; additionally anaerobic methanogenesis processes with high organic loads consume many protons leading to increased alkalinity within the system thus enhancing absorption capacit for CO2 into liquid phase. This further facilitated the reduction of CO2 content and the elevation of CH4 content in landfill gas.