Taking four maize inbred lines with different photosynthetic rate and their two hybrids as test materials, the diurnal variations of their photosynthesis parameters in silking stage were measured to study the heterosis of photosynthetic performance. The results showed that the net photosynthetic rate (P), transpiration rate (Tr) and stomatal conductance (Gs) all presented obvious single-peaked curve in a day, with the peak value occurred at 10:00-12:00, 12:00 and 10:00-12:00, respectively, while water use efficiency (WUE) had a "V" type variation trend, with the lowest value appeared at 12:00. The diurnal variation of Pn and Tr correlated markedly with Gs, suggesting that Gs played an important role in regulating the diurnal variation of Pn and Tr. Pn, Tr and Gs had higher heterosis in the afternoon than in the morning, while WUE was in reverse, indicating that maize hybrid had higher resistance to high temperature and dehydration in the afternoon, which provided a new path to select varieties with high net photosynthetic rate.