Under global warming and water resource constraints, winter wheat yield in the Haihe Plain has continued to increase despite decreased agricultural water use, reflecting enhanced adaptation of newer cultivars to water limitation. However, the mechanisms through which roots adapt to water limitation and enhance water productivity (WP_c) remain unclear. This study, based on 25 major cultivars since 1950s, analyzed root characteristics and their relationship with WP_c over three years of field and tube experiments. Results showed that with cultivar replacement, the number of tillers and secondary roots significantly decreased by 0.66% and 0.55% y⁻¹, respectively, while primary roots increased by 0.56% yr⁻¹. In the field, root mass in the 0–0.4 m soil layer decreased significantly, with root mass density (RMD) and root length density (RLD) decreasing by 0.0011 mg cm⁻³ yr⁻¹ and 0.014 cm cm⁻³ yr⁻¹, respectively, while root activity (RA) increased by 0.237 μg g⁻¹ h⁻¹ yr⁻¹. Tube experiments showed a shift towards deeper roots, particularly in the 1.2–1.6 m soil layer. Yield increased by 57.35 kg hm⁻² yr⁻¹ due to higher grain weight, while yield stability and water productivity (improved by 0.66% yr⁻¹) also enhanced during cultivar replacement. Field shallow RLD and RMD were negatively correlated with yield and WP_c, whereas tube post-anthesis deep RLD, RMD, and RA were positively correlated with grain weight, yield, and WP_c. Overall, cultivar replacement reduces shallow-root investment while enhancing RA, whereas deep-root may improve WP_c by sustaining grain-filling water uptake, delaying senescence, and increasing grain weight, though deep-root trends require field validation.