Abstract: Aiming at the effects of high humidity and drought on grain filling and yield reduction of summer maize, and revealing its physiological mechanism to disaster, the authors designed 3 treatments of high humidity, drought and control of soil in the summer maize growing season by artificially controlling soil moisture in 2016, and measured the diurnal variation of photosynthetic characteristics, light saturation point, light compensation point and dark respiration of leaves at night at the grain filling stage. The results showed that: the diurnal variation curve of the net photosynthetic rate of maize leaves was bimodal, that was“M”, while that of the high humidity treatment was weakened into a single-peak pattern, and that of the drought treatment continued to decline throughout the day; the diurnal variation law of stomatal conductance (Gs) and transpiration rate (Tr) was consistent and they had a significant positive correlation; while that of intercellular CO2 concentration (Ci) and water use efficiency (WUE) showed a negative correlation; the dark respiration under the high humidity treatment increased by 0.5800-0.7767 μmol CO2/(m2 ·s) compared with that of the control, while that of the drought treatment reduced by 0.1767-0.4933 μmol CO2/(m2·s) compared with that of the control, the reason for this situation might be that the high humidity condition led to the intensified respiration process of maize leaves and other green organs, which resulted in the decrease of nutrient accumulation in grains and ergo the drop of yield; according to the analysis of light saturation points and light compensation points, the interval of photosynthesis effectively using light under high humidity treatment expanded, while under drought stress, that value went down, which might be the main physiological reason for the reduction of maize yield resulting from drought at the grain filling stage.