Numerous data demonstrate that global climate change affects crop yield and quality. However, the effect of climate change on the nutritive value of forage crops remains unclear. Oat grass (Avena sativa L.) was used as a representative forage crop to understand changes in yield and chemical composition. The growth of oat grass under elevated temperature and CO2 conditions was simulated in a 51-day growth experiment inside OTCs. The oat grass was harvested during the heading period and yield, nutrient content, macro- and micro-mineral content and in vitro dry matter digestibility (IVDMD) determined. The fresh and dry yield, mineral P and Fe content and IVDMD of oat grass increased under increased CO2 concentrations (P < 0.05). As temperature increased, dry matter yield and soluble sugar content decreased and the content of Na, Mg, P, S, Ca, Mn, Fe, Cu and Zn increased (P < 0.05), while IVDMD was not significantly affected. Under the interaction of increasing CO2 and temperature, the content of Mg, P, S, Ca, Mn and IVDMD of oat grass increased (P < 0.05); however, there was no significant effect on yield. Climate change may result in oat grass containing more digestible nutrients and minerals for ruminants.