CONTEXT: Climate change affects water resources and soil fertility in northern China. Hence, understanding the drivers and relationships between spring maize water use efficiency (WUE) and soil fertility is critical. OBJECTIVE: To assess how spring maize WUE responds to changing soil fertility in northern China under future climate scenarios and propose region-specific strategies. METHODS: Three CMIP6 circulation models were coupled with the validated DSSAT-CERES-Maize model to simulate WUE and soil organic carbon (SOC) from 2021 to 2100 under baseline, SSP126, SSP245 and SSP585 scenarios. RESULTS AND CONCLUSIONS: WUE increased under SSP126 (1.90 kg m(-3)) and SSP245 (1.84 kg m(-3)), but declined under SSP585 (1.67 kg m(-3)), compared to the baseline (1.75 kg m(-3)). SOC showed a notable increase until 2070, then stabilized. Spatially, WUE ranked northeast China (NE) > northwest China (NW) > north China (NC), while SOC followed the pattern NE > NC > NW, with reductions averaging 12.5-18.2%. In NC, WUE peaks at moderate soil fertility and remained stable; in NE, it steadily increased with increasing soil fertility; and in NW, WUE increased to good fertility then declined. Yield dominated WUE at moderate and above fertility, while evapotranspiration was the primary driver under poor fertility in NW and NC. SIGNIFICANCE: These findings highlight that future WUE improvement depends on region-specific management. In NE, enhancing SOC through organic-inorganic fertilization can effectively boost yield and thus raise WUE. In NC and NW, WUE is improved by reducing evapotranspiration through water-saving practices.