Field experiments of eight summer maize cultivars with three plant density degree were conducted in Fengqiu Experimental Station of Chinese Academy of Sciences from 2008 to 2010． The object of the study was to assess cultivar and plant density effects on summer maize grain yield and water use efficiency ( WUE) ，and with optimization of cultivar and density，to explore yield and WUE potential of summer maize． Cultivar and plant density affected maize grain yield significantly with difference between experiment seasons． There was 1 050 － 1 500

kg /ha difference between maximum and minimum grain yield affected by cultivar difference． Different cultivar grain yield response differently to plant density degree，and the grain yield of 60 000 － 67 500 plant /ha density was higher than 75 000 plant /ha density． The sunshine during maize growth affect grain yield significantly，and compared with annual value，sunshine hour decreased 56． 6%，with corresponding 15% grain yield reduction． Effective light radiation of the leaves above ear has a positive relativity to grain yield． The results showed cultivar with higher grain yield had a higher WUE，and selecting higher yield cvivar is an important approach to promote WUE of summer maize．