In order to meet the requirements of low-carbon economy, the reduction and utilization of global-warming gases have become the focus of attention. There are a large amount of toxic and harmful gases being emitted from the combustion of fossil fuels and syngas,as well as being released during mining activities. It is therefore necessary to trap and separate these gases for eliminating their environmental contamination,also increasing the feedstocks for industrial production. For avoiding the contamination and safety problems raised from the conventional alkali absorbents, the ionic liquids,namelyliquid molecular sieve,have become green alternatives to absorb and separate different gases. Accordingly, the research progress about separating gases by pure ionic liquids is reviewed in this paper. The influencing factors including temperature,pressure, the properties of gas and ionic liquids,and the functional groups of ionic liquids, were analyzed. It is stated that the ionic liquids with alkali groups on cation and anion are promising CO_2 solvents and competent separator for removing CO_2 from some sparsely soluble gases. In order to separate CO_2 from strongly acidic SO_2 and H_2S,moderate basicity exhibits much better separating ability because strong alkali group could strongly bind with both SO_2(or H_2S) and CO_2, leading to low SO_2 / CO_2 and H2S / CO_2 selectivities. Ionic liquids with small molecular weights and compact structures usually perform better selectivity,and low temperature and pressure are favorable for increasing the separation. Additionally,combining experimental results and molecular dynamic simulation, the real selectivity of binary gases in ionic liquids was calculated based on Equations of State. It is concluded that feed gas ratio, ionic liquid contents, temperature and pressure are significant factors required to be adjusted to approach a great real gas selectivity.