Abstract: This paper deals with the characteristic of diurnal courses of photosynthesis and the reasons of midday photosynthetic depression without lack of moisture in soil for two main community-forming species, A. chinense and S. grandis in a typical steppe zone. The results of research show that the diurnal courses of photosynthesis of these two species are of the highpeak type before noon and their photosynthetic rate at midday decreases certainly. The main reasons for the depressing photosynthesis are the decrease of leaf water content and the stomatal conductance caused by the fall of atmospheric humidity. A secondary effect is the increment of light respiration resulting from the rise of air temperature and light intensity in the midday period. The reduction of CO2 concentration, and the accumulation of photosynthetic product also contribute to photosynthetic depression at midday. Biorhythm has no effect on photosynthetic depression during the midday period.