Introduction Moderate densification and fertilization are widely used in agricultural practice due to their advantages in improving crop population structure, but it is currently unclear whether the organic combination of the two can optimize crop water consumption characteristics and improve water use efficiency (WUE).Methods A split-plot design, which contained the main plot setting three fertilization levels: N1 (organic fertilizer only), N2 (inorganic fertilizer only), and N0 (no fertilizer) and the sub-plots setting three densification levels: D1 (200,000 plants hm(-)& sup2;), D2 (400,000 plants hm(-)& sup2;), and D3 (600,000 plants hm(-)& sup2;), was employed to investigate the combined effects of the two factors on the water use characteristics in foxtail millet.Results The results showed that N2D2 significantly increased the leaf area index (LAI), aboveground biomass (AB) and root biomass (RB) of foxtail millet, and the grain yield was increased by 0.75% - 38.62% compared with other treatments. Meanwhile, the N2D2 treatment significantly reduced soil evaporation (E), increased plant transpiration (T), and raised the transpiration/evapotranspiration ratio to 61.44% - 62.03%. The soil water storage (SWS) capacity remained at a relatively high level in the 0-100 cm soil layer. Ultimately, the WUE of this treatment was significantly increased by 3.41% - 35.64%, and the water consumption structure was optimal. The structural equation model further revealed that the increase in WUE was mainly attributed to the interaction effect of fertilization and density, which promoted root biomass (RB) increase by optimizing SWS in the 0-40 cm soil layer, thereby influencing AB to positively regulate WUE.Discussion In conclusion, under the condition of chemical nitrogen fertilizer application, moderate densification (400,000 plants hm(-)& sup2;) optimizes the population structure and water consumption patterns, synergistically enhancing both crop yield and water use efficiency. This approach represents an effective agronomic practice for achieving high-yield and water-saving cultivation of foxtail millet in arid regions.