The temperature increase due to the greenhouse gases emission is the hot topic of international community. By using the IPCC SRES A2 and B2 greenhouse gases emission scenarios, regional climate model- PRECIS and crop model- CERES, combined with the social economical scenarios downscaled from the IPCC SRES, the grain supply and demand in 2020, 2050 and 2080 were simulated and analyzed. The Results show that the three main food plants yields would decrease under both A2 and B2 scenarios without considering the CO₂ direct effect, the range of yield descent would reduce if there is enough irrigation. Yield could increase yet if the CO₂ direct effect was included in the simulation. With the assumption of a 5% grain import percentage, a population increase scenario described by SRES A2 or B2, technology advance will make grain yield increase above 0.7% or 1% by 2030. With the same percentage of plants and planting areas as present, food supply would meet the demand under B2 scenarios, while would not under A2 scenarios.