Optimizing winter wheat population structure under sub-ETc drip irrigation is critical for sustainable intensification in saline-prone arid regions. We conducted a three-year field experiment (2022–2025) in northwestern China on mildly saline-alkaline soil (EC_1:5 0.84–0.95 dS m⁻¹; ECe 4.2–4.8 dS m⁻¹), testing three seeding densities (D1: 150, D2: 225, D3: 300 kg ha⁻¹) under three drip irrigation regimes (W0: 90% ETc, regional conventional baseline; W1: 80% ETc, mild deficit; W2: 70% ETc, moderate deficit) without plastic mulch. We note that W0 does not include an explicit leaching fraction and therefore does not represent a long-term leaching-compliant reference. Root length density (0–100 cm) was quantified using WinRHIZO, and partial least squares path modeling (PLS-PM) elucidated mechanistic associations. Under W1, the tiller-reliant high-density strategy (D3) experienced 34.8% tiller mortality, primarily associated with surface salt accumulation (r = 0.82, P < 0.001; 0–20 cm EC_1:5 rose 18.9% to 1.13 dS m⁻¹), with secondary contributions from light competition (β = 0.28, P < 0.05). The main-stem-dominant low-density strategy (D1) maintained 91.5% productive spike survival, exhibiting 37.5% higher RLD in 60–100 cm (0.55 versus 0.40 cm cm⁻³) corresponding with less-saline subsoil (EC_1:5 0.72 dS m⁻¹ at 80–100 cm). PLS-PM identified deep-root proliferation as the strongest mediator (path coefficient 0.68, P < 0.001) of post-anthesis photosynthesis; stay-green was extended by 5.2 d under D1W1. Among the nine treatments tested, D1W1 emerged as the best-performing combination, achieving stable three-year mean yield of 7.92 t ha⁻¹ (CV = 4.3%) and the highest irrigation water productivity (IWP = 2.07 kg m⁻³). Although 5.8% lower than D3W0 (8.41 t ha⁻¹), D1W1 yielded equivalent net returns (15,850 RMB ha⁻¹ ≈ 2217 USD ha⁻¹) by saving 50% seed and 11% water.