Severe wintertime PM_2.5 pollution persists in the Urumqi–Changji–Shihezi urban agglomeration, China's largest arid-region urban cluster, despite emission controls that have improved air quality in eastern China. Using air quality monitoring data, radiosonde observations, and chemical transport model simulations for two winters (2022–2024), we investigated the combined roles of synoptic-scale circulation and basin terrain in modulating pollutant accumulation. Results show that PM_2.5 pollution exhibits strong spatial synchrony among Urumqi, Changji, and Shihezi, indicating dominant regional meteorological control beyond local emissions. Stagnant westerly circulation with warm advections aloft fosters persistent cold-air pool within the Junggar Basin, producing strong thermal inversions, shallow boundary layers, and PM_2.5 accumulation in the urban agglomeration. In contrast, Siberian cold surges and northwesterly flows channel cold air through low-elevation gaps on the basin's north and west margins, weakening stratification, enhancing surface winds via horizontal and partial downward momentum transport, and deepening the boundary layer, which promotes pollutant dispersion. These findings highlight the critical interplay between large-scale forcing and complex terrain in controlling air quality in arid basins and provide a scientific basis for improved forecasting and targeted mitigation strategies.