Abstract: In this study the effects of sunny and overcast days during dry season in central Yunnan province on photosynthetic characteristics of four afforestation tree species (Pinus yunnanensis, Alnus nepalensis, Quercus acutissima, Cyclobalanopsis glaucoides) in juvenile pure and mixed plantations were investigated, and the responses of net photosynthetic rate (Pn), transpiration rate (Tr), water use efficiency (WUE) and carboxylation efficiency (CE) for each tree species in different type juvenile plantations to different weather conditions were analyzed. The results show that Pyunnanensis and Anepalensis present higher Pn, Tr and CE on sunny days but higher WUE on overcast days. Cglaucoides present higher Pn, WUE and CE on overcast days but higher Tr on sunny days. The responses of photosynthetic characteristics of Qacutissima to sunny and overcast days are closely related with plantation types. Different mixed patterns improve Pn, Tr, WUE and CE of Pyunnanensis in sunny and/or overcast conditions. Different mixed patterns show better stimulation effects on Pn and WUE of Anepalensis in overcast conditions. Different mixed patterns have certain inhibition effects on Pn, Tr and CE of Cglaucoides in sunny and/or overcast conditions, while which slightly increase WUE of Cglaucoides in overcast conditions.