【Objective】The objective of this study was to identify exogenous spermidine on photosynthesis and chlorophyll fluorescence under high temperature treatments during flowering (0–8 d before silking and 0–8 d after silking).【Method】The heat-tolerant maize genotype XD20 and heat-sensitive maize genotype ZY309 were used in this study. Photosynthetic parameters, chlorophyll fluorescence variation, the activities of ribulose-l,5-bi-sphosphate carboxylase (PEPCase) and phosphoenolpyrovate carboxylase (RuBPCase) were investigated during flowering.【Result】The results showed that the heat-sensitive genotype got a significant reduction in grain yield with a more reduction rate under the treatment after flowering than before flowering. It was less affected for heat-tolerant genotype. SPAD, net photosynthetic rate (Pn), stomatal conductance (Gs), primarily photochemical efficiency (Fv/Fm), actual photochemical efficiency (?PSII), photochemical quenching (qP), PEPCase and RuBPCase activities of ear-leaves were reduced. But intercellular CO2 concentration (Ci), non-photochemical quenching (qN) were increased.【Conclusion】The results showed that there were significant effects of high temperature treatments during flowering on photosynthetic characteristics of summer maize, and the yield were significantly decreased. The heat-tolerant maize genotype has more photosynthesizing capacity and chlorophyll content than the heat-sensitive maize genotype.