Response of eco-physiological characteristics to CO₂ in Stipa bungeana,Artemisia sacrorum,Stipa grandis,and Aneurolepidium dasystachys leaves was studied.Results showed that with CO₂ concentration increasing,photosynthesis rate(Pn),intercellular CO₂ concentration,and water use efficiency(WUE) of the four species were increased and stomatal conductance and transpiration rate(Tr) were decreased.Photosynthesis characteristics of the four species varied with season.The order of the species,in terms of Pn and Tr,was: Stipa grandis Artemisia sacrorum Stipa bungeana Aneurolepidium dasystachys;and the order of the species,in terms of WUE,was: Stipa bungeana Artemisia sacrorum Stipa grandis Aneurolepidium dasystachys.Meanwhile,PSⅡ open proportion,conversion efficiency of primary light energy under light of PSⅡ,actual PSⅡ efficiency,and PhiCO₂ were linearly increased with CO₂ concentration.However,the non-photochemical quenching coefficient presented a decline trend.