Selecting an appropriate planting density is an effective way to improve crop water productivity (WPC). However, there is a lack of research on the balance between evapotranspiration (ET) partitioning, water consumption, and grain production under different summer maize planting densities. To close this knowledge gap, a two-year field experiment was conducted in the North China Plain (NCP) to reveal the effects of different planting densities (HD: 100,000 plants ha-1; MD: 78,000 plants ha-1; LD: 56,000 plants ha-1) on ET partitioning, grain yield, and water productivity of summer maize. The water-heat-carbon-nitrogen simulator (WHCNS) model was employed to calculate ET partitioning and perform scenario simulation after calibration and validation. The results showed that compared to the LD treatment, ET of the summer maize and grain yield in the MD and HD treatments significantly increased. Model simulations showed that the ratio of evaporation to ET ranged from 25.6% to 30.7% and reduced as increasing planting densities. Increasing planting density enhanced total transpiration of summer maize more than 20 mm, comparing to LD treatment, and the most significant differences between various planting densities appeared at the mid-growth stage (August 1 to 31). Scenario simulations indicated that grain yield and WPC of summer maize were consistently higher in wet and normal years compared to drought years, exhibiting a trend of initially increasing and then decreasing with increasing planting density. The highest grain yield and WPC of summer maize were observed at a planting density of approximately 80,000 plants ha-1. The results provide theoretical support for selecting a summer maize planting density and effectively utilizing agricultural water in the NCP. (c) 2025 Crop Science Society of China and Institute of Crop Science, CAAS. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NCND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).