Air transport network accessibility serves as a critical indicator for evaluating the service capacity of aviation systems, closely linked to factors such as route network design, airport infrastructure, transfer efficiency, and fleet allocation. This study proposes an Improved Supply-Side-Oriented Air Transport Network Accessibility Model (ISSO-ATNAM), which overcomes the traditional connectivity indices' overreliance on supply scale. A comprehensive evaluation framework integrating network coverage, flight density, and transfer connectivity is established. Based on flight schedules, the model prioritizes identifying optimal paths and minimum travel time between origin–destination (OD) pairs under transfer-based travel modes, computing accessibility indices for origin airports and transfer efficiency indices for transit airports. By optimizing network design, the supply-side accessibility framework enhances OD connectivity through transfers, thereby identifying potential airports and improving network synergy. Focusing on Xinjiang—a vast, geographically complex region in northwestern China, ISS-ATNAM is applied to evaluate intra-regional connectivity improvements through empirical case studies. Analysis in Xinjiang demonstrates the effectiveness of the “core hub + branch network” strategy: from 2015 to 2024, the number of airports increased by 68.75%, total OD pairs expanded by 150%, and the accessibility index achieved an annual growth rate of 14.58%. Urumqi Airport significantly strengthened its hub radiation capacity, while branch airports like Turpan exhibited breakthrough accessibility growth, and nodes such as Yining demonstrated potential as secondary hubs. The findings provide theoretical support for aviation network planning in frontier regions and empirical insights for China's “trunk-branch-connection” civil aviation strategy.