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Field experiments were conducted from 2012 to 2015 in an arid region of Northwest China to investigate the effects of planting density on plant growth, yield, and water use efficiency (WUE) of maize for seed production. Five planting densities of 6.75, 8.25, 9.75, 11.25 and 12.75 plants/m(2) were conducted in 2012, and a planting density of 14.25 plants/m(2) was added from 2013 to 2015. Through comparison with the AquaCrop yield model, a modified model was developed to estimate the biomass accumulation and yield under different planting densities using adjustment coefficient for normalized biomass water productivity and harvest index. It was found that the modified yield model had a better performance and could generate results with higher determination coefficient and lower error. The results indicated that higher planting density increased the leaf area index and biomass accumulation, but decreased the biomass accumulation per plant. The total yield increased rapidly as planting density increased to 11.25 plants/m(2), but only a slight increase was observed when the density was greater than 11.25 plants/m(2). The WUE also reached the maximum when planting density was 11.25 plants/m(2), which was the recommended planting density of maize for seed production in Northwest China.