Abstract: Leymus chinensis is a dominant, rhizomatous perennial C₃ species found in the grasslands of Songnen Plain, Northern China, where its productivity has decreased year by year due to environmental changes. As a dominant species growing in arid and semiarid regions, precipitation is a key factor limiting plant productivity. To determine how this species’ productivity responds to different precipitation regimes and increased CO₂ levels, we conducted an experiment that measured photosynthetic parameters, along with the accumulation and partitioning of biomass. Plants were subjected to combinations of three precipitation gradients (normal precipitation, normal±40%) and two CO₂ levels (380±20 and 760±20 μmol/mol) in environment-controlled chambers. The interaction between increased CO₂ and precipitation had significant effects on photosynthetic parameters, belowground biomass and root∶shoot ratio, but no effect on aboveground biomass. Net photosynthetic rate, stomatal conductance, transpiration rate, aboveground and total biomass rose with increases in precipitation and CO₂ concentration, but no significant change was observed when precipitation increased from normal to high under CO₂ enrichment. There was no significant difference in the ratio of root to shoot among precipitation regimes at the low CO₂ condition, but it changed significantly with CO₂ enrichment and low precipitation. Water use efficiency and intercellular CO₂ concentration increased significantly when precipitation was low, but decreased when precipitation was high under CO₂ enrichment. The effect of elevated CO₂ on photosynthesis and biomass accumulation was more obvious at low precipitation than at normal or high precipitation. The results suggest that at ambient CO₂ levels, net photosynthetic rate and biomass of L. chinensis increase with precipitation, but that these measures are not further affected by additional precipitation when CO₂ is elevated. Furthermore, CO₂ may partly compensate for the negative effect of low precipitation on the growth and development of L. chinensis.