Freshwater scarcity severely limits sustainable cotton production in arid regions. This study aimed to establish the optimal salinity threshold for staged saline water supplemental irrigation (SWSI) and elucidate its canopy-level mechanisms in optimizing within-boll yield components and fiber quality. A two-year field trial (2023-2024) was conducted in Awati County, Xinjiang, using mulched drip irrigation at five SWSI levels (3.5-9.5 g L-1) and a freshwater control (CK). Compared with CK, 3.5 g L-1 treatment significantly increased lint yield by 31.4%, boll number per plant by 22.45%, and fibers per seed by 6.01-10.59%, while fiber length and strength rose by 6.98-10.38% and 2.69-6.00%, respectively. When salinity reached 8.0 g L-1, yield declined by 8.5%, and a salinity of 9.5 g L-1 reduced yield by 24.52%. Spatially, mid-fruiting branches (nodes 4-6) remained stable, maintaining high lint mass per seed even under high salinity, whereas upper branches (>= node 7) were most sensitive; at 9.5 g L-1, the boll number (0.36) was 56.6% lower than at 3.5 g L-1 (0.83), and the Q-score decreased by 6.7%. These results demonstrate that SWSI with <= 5.0 g L-1 salinity (optimum 3.5 g L-1) simultaneously enhances lint yield and fiber quality, providing a practical strategy for efficient saline water use in arid cotton regions.