In order to study the photo-physiological characteristics of old world bluestem (Bothriochloa ischaemum (L.)Keng.)from loess hilly-gully region under the background of global climate change,the chlorophyll content(SPAD)and photosynthesis light response curves were determined under potted conditions with different atmospheric CO_2 conditions,soil moisture conditions and nitrogen deposition treatments. The results showed that elevated CO_2 and nitrogen deposition increased the SPAD value,maximum photosynthetic rate (P_(max)),apparent quantum efficiency (AQE),light saturation point(LSP); drought stress decreased the SPADvalue,Pmaxand LSP.Multi-factor analysis of variance indicated that CO2concentration,soil water conditions and nitrogen deposition level all had highly significant influence on SPAD value,Pmaxand LSP;and nitrogen deposition level had significant influence on AQE.CO_2 concentration and soil water conditions had significant interaction effects on P_(max),AQEand LSP;CO_2 concentration and nitrogen deposition level had significant interaction effects on Pmax,AQEand light compensation point(LCP);soil water conditions and nitrogen deposition levels had significant interaction effects on P_(max),AQEand LSP.RDA analysis indicated that nitrogen was the most important factor that affected the photosynthesis characteristics of B.ischaemum.Elevated CO_2 concentration and nitrogen deposition had compensation effect on the photosynthetic reduction of B.ischaemuminduced by drought stress under the background of global climate change.