The main objective of this research was to analyse the effects of plant population on the dynamic changes of photosynthetic characteristics, soluble sugar content, carbon and nitrogen in corn leaf (Zea mays L.). Field experiments were conducted in Gongzhuling, Jilin province. Three corn hybrids were cultivated at 60 000, 75 000, 90 000, and 105 000 plants ha^-1. Treatments were arranged in a split-plot design with three replicates. Plant density was the main-plot and hybrids were the subplot. The chlorophyll content, photosynthetic characteristics, contents of carbon and nitrogen in corn leaf were measured at different stages, especially after silking. The results indicated thatthe chlorophyll content, the photosynthetic rate in grain filling stage, soluble sugar content, leaf total nitrogen decreased in all hybrids with the increase of plant density. It showed that transmission rate reduced, the leaf senescence accelerated, physical activity declined as plant density intensified. During the growth period, chlorophyll content was changed in a single peak curve, with the peak at grain filling. The peaks of carbon-nitrogen ratio appeared at silking and ripening respectively. The chlorophyll content, photosynthetic rate, soluble sugar content, ratio of carbon to nitrogen in corn leaf indicated a quadratic relationship with increasing days after silking. Total nitrogen in corn leaf decreased linearly in all hybrids as days after silking increased. High plant density had great effect on carbon metabolism. The chlorophyll content had no or little effect on the photosynthetic rate after silking. The results showed the dynamic changes of photosynthetic characteristics, contents of carbon and nitrogen were different as plant density intensified. The tolerance to planting density is Xianyu 335>Zhengdan 958>Jidan 209.