The determination of critical thresholds' is an essential task for informed policy decisions on establishing greenhouse emission targets. The threshold of temperature increase due to greenhouse gas enrichment has become the key point of negotiation of climate change. By using the A2 and B2 climate scenarios produced by Regional Climate Model PRCIS, modified crop model-CERES, the threshold of temperature increase under climate change for Chinese food production and its uncertainties were simulated and analyzed based on the yield variation of three main food crops (rice, wheat and maize). Result shows: there is no temperature increase threshold if the interactive effects of temperature increase and elevated CO₂ concentration were considered in the analysis; the temperature increase threshold emerges in 2℃ if only the effect of temperature increase was included in the simulation; the adaptation can mitigate the negative effect of climate change on grain production. If irrigation pushes the above-mentioned threshold to 2.5℃, early sowing and changing to middle-growth cultivars increase the yield and decrease the yield variability, and modify the threshold also. There are three kinds of uncertainties in this assessment: scenarios, methods, and adaptation. The effects of CO₂ enrichment and climate change need further study.