In semi-arid dryland systems on the Loess Plateau, rainfall is highly variable within the season and drought frequently occurs around drought-sensitive stages, making stage-targeted micro-supplemental irrigation essential for stabilizing sorghum yield. However, the optimal timing and allocation of micro-supplemental irrigation, and the root-nitrogen mechanisms linking stage-targeted water inputs to yield formation and water use efficiency (WUE), remain insufficiently quantified. To address these gaps, a two-year field experiment (2022-2023) was conducted in a National High-Standard Farmland, Loess Plateau. Based on regional precipitation patterns (1957-2017), locally available irrigation water for sorghum, and crop water-demand characteristics, the experiment included single- and dual-phase micro-supplemental irrigation treatments, along with rainfed controls (CK0: no mulch; CK1: mulched). Single-stage irrigation included two jointing-stage treatments (15 mm: I15-0; 24 mm: I24-0) and one tasseling-stage treatment (15 mm: I0-15). Double irrigation was applied at both stages: 15-15 mm (I15-15), 24-15 mm (I24-15), 9-15 mm (I9-15), and 15-9 mm (I15-9). Relative to CK0 and CK1, I15-15 significantly increased sorghum yield by 98.1 %-138.4 %, WUE by 63.7 %-95.1 %, and harvest index by 17.4 %- 34.3 %. It also increased grain starch content by 0.9 %-10.5 % and changed grain fat content by -5.3-13.4 %, while reducing protein content by 7.7 %-18.8 %. In addition, I15-15 reduced NO3--N residues in the 0-100 cm soil layer by 33.6 %-47.2 %, concurrent with higher aboveground nitrogen accumulation (9.6 %-77.3 %). Structural equation modeling and Random Forest analyses indicated that micro-supplemental irrigation improved root morphology (higher root length density and root surface area density), which in turn enhanced nitrogen uptake, leaf area index, and chlorophyll content, ultimately leading to increased yield and WUE. Therefore, applying micro-supplemental irrigation at both jointing and tasseling is recommended for dryland sorghum in semi-arid regions to maximize yield and WUE while limiting post-harvest soil nitrate accumulation. This study quantifies an actionable split irrigation strategy for dryland sorghum and, based on multiple analytical approaches, demonstrates that improvements in root morphology constitute a key pathway linking limited water inputs to enhanced nitrogen uptake, canopy function, and yield.