Taking high-yielding summer maize cultivars Zhengdan 958 (ZD 958) and Denghai 661 (DH 661) as test materials, this paper studied the effects of different planting density (22500, 45000, 67500, 90000, and 112500 plant x hm(-2)) on the grain yield and source-sink characteristics of the cultivars in four regions, i.e., Shandong Agricultural University, Wenkou, Yanzhou, and Laizhou. At the planting density 112500 plant x hm(-2), the grain yield and the biomass of the cultivars were the highest, being 19132 and 36965 kg x hm(-2), respectively. Comparing with those at the planting densities 22500 and 67500 plant x hm(-2), the grain yield at the planting density 112500 plant x hm(-2) was increased by 72% and 48%, and the biomass was increased by 152% and 112%, respectively. With the increase of planting density, the leaf area index (LAI) increased significantly, while the leaf area per plant (LA), the maximum number of filaments, the grains per ear, and the 1000-grain weight all decreased. The harvest index and the grain-leaf ratio decreased with increasing planting density, but no significant change was observed when the planting density was higher than 67500 plant x hm(-2), suggesting that at higher planting densities, summer maize could improve their grain yield via increasing population sink.