A field experiment was conducted to study the effects of increasing field temperature on the grain yield and photosynthetic characteristics of summer maize cultivars ND108 and YD13. The results showed that under increasing field temperature in the period from male tetrad to maturing stage, the grain yield of ND108 and YD13 decreased by 46.6% and 45.1%, leaf area index (LAI) decreased by 15.4% and 11.5%, and net photosynthetic rate of ear leaf decreased by 22.85% and 18.14%, respectively, compared with the control. The contents of leaf chlorophyll a and b, especially that of chlorophyll a, decreased obviously, and the activities of ribulose-1,5-bisphosphate carboxylase (PEPCase) and phosphoenolpyrovate carboxylase (RuBPCase) in the leaves of the two cultivars decreased by 51.1% and 32.4%, and 29.5% and 7.7%, respectively.