Drought and soil salinization are major environmental constraints limiting cotton productivity in arid regions, necessitating synergistic strategies targeting water regulation, rhizosphere microecological optimization, and plant physiological adjustment to achieve high-quality production. A two-year field experiment (2023-2024) was conducted in a typical arid saline-alkali region of Xinjiang, China, to systematically evaluate the synergistic effects of magnetoelectric brackish water irrigation combined with organic fertilizer and plant growth-promoting rhizobacteria (PGPR, Bacillus subtilis) application rates (15, 22.5, 30, 37.5, and 45 kg ha(-1), labeled as RB1-RB5) on cotton growth dynamics, rhizosphere bacterial community structure, stress resistance, yield, and fiber quality formation. Magnetoelectric brackish water irrigation significantly alleviated salinity-induced constraints on the rhizosphere environment, increased bacterial diversity, and optimized community composition. On this basis, moderate PGPR application further reshaped rhizosphere bacterial composition, enhanced co-occurrence network connectivity and complexity, and significantly increased the functional potential of inorganic nitrogen assimilation and nitrogen transport, without inducing excessive activation of inefficient nitrogen transformation pathways such as denitrification and DNRA. Optimization of the rhizosphere microecology significantly improved cotton growth dynamics, as evidenced by simultaneous increases in maximum plant height (H-m) and growth rate parameters (HVmax and HVavg), and by prolonged rapid growth duration that promoted biomass accumulation, resulting in plant height increases exceeding 19.5% (P < 0.05). Meanwhile, PGPR application markedly enhanced leaf antioxidant enzyme activities, including superoxide dismutase, peroxidase, and catalase, while reducing malondialdehyde content, thereby alleviating salinity-induced oxidative damage. Mantel tests and partial least squares path modeling revealed that the coupled regulation indirectly enhanced water and nitrogen use efficiency and biomass allocation efficiency through a coordinated pathway of "rhizosphere microbial community-growth dynamics-antioxidant metabolism," thereby driving the synergistic formation of cotton yield and fiber quality. Seed cotton yield increased by 7.7-29.0% and 5.0-21.2% in 2023 and 2024, respectively, while the fiber quality index increased by 18.5-46.0% and 21.2-48.4%. Quadratic regression analysis further demonstrated significant nonlinear promotion-inhibition responses of plant height, dry matter accumulation, yield, and fiber quality index to PGPR application rate, with an optimal range of 30-38 kg ha(-1) that maximized the combined benefits of yield stability and quality improvement. Overall, magnetoelectric brackish water and moderate PGPR application jointly established a regulatory mechanism for stable and high-quality cotton production in arid saline-alkali regions. This study provides a synergistic management strategy with explicit mechanistic support for the safe and efficient utilization of brackish water and sustainable cotton production in saline-alkali arid regions.