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• Reduced irrigation decreased SWC, LAI, biomass, yield and ET, but improved WUE. • Increasing planting density increased LAI and ET, but reduced SWC and biomass. • Planting density had significant effects on yield and WUE. • MI-D2 is recommended for spring maize production in the Northwest China. The limited availability of water resources severely restricts agricultural development in the semiarid and arid regions of Northwest China. A two-year field experiment was conducted to investigate the effects of irrigation and planting density on the soil water content (SWC), growth and development, yield, water use efficiency (WUE) and economic benefits of spring maize in arid areas under mulch drip irrigation system. The optimized combination of irrigation and planting densities for high grain yield, WUE and net return were obtained by multiple regression analysis. Experiment included three irrigation treatments (HI: 450 mm; MI: 337.5 mm; LI: 225 mm) and four planting densities (D1: 75,000 plants ha−1; D2: 90,000 plants ha−1; D3: 105,000 plants ha−1; D4: 120,000 plants ha−1). The results showed that the reduction of irrigation decreased SWC, leaf area index (LAI), aboveground biomass, yield and evapotranspiration (ET), but improved WUE. HI and MI were significantly different from LI. Increasing planting density increased LAI and ET, but reduced SWC and aboveground biomass. Planting density had significant effects on yield and WUE, which differed in both years. The overall yield, WUE and net return were greater than the two-year average of HI-D1 under an irrigation amount of 271.9–500.2 mm and a planting density range of 7.1–14.3 × 104 plants ha-1. MI-D2, with 25 % reduced irrigation, increased the two-year average yield, WUE and net return by 5.2 %, 20.2 % and 27.2 %, respectively, compared to HI-D1. Therefore, irrigation of 337.5 mm at a medium planting density of 90,000 plants ha-1 under mulch drip irrigation is recommended for spring maize production in the drylands of Northwest China in our research. [ABSTRACT FROM AUTHOR]