Efficient　CO2/CH4　separation　for　biogas　upgrading　can　be　achieved　by　using　ionic　liquids　(ILs)　as　absorbents.　In　this　study,　a　novel　index　named　＂relative　absorption　ability＂　(RAA)　was　proposed　for　the　first　time　to　facilitate　the　screening　and　evaluation　of　potential　absorbents　based　on　their　thermodynamic　and　kinetic　properties　for　CO2/CH4　separation.　Subsequently,　the　properties　of　2009　ILs　were　predicted　with　the　conductor-like　screening　model,　and　the　top　10　ILs　were　screened.　Finally,　the　cost　of　the　screened　top　two　ILs　for　CO2/CH4　separation　was　further　estimated　with　process　simulation.　The　results　further　prove　the　reliability　of　the　new　proposed　RAA　index　in　screening　and　evaluating,　and　the　screened　promising　ILs　can　reduce　the　costs　of　CO2/CH4　separation　by　53.9%　and　51.9%,　respectively,　in　comparison　to　the　current　commercial　technology.