The Sichuan Basin (SCB) is located in southwestern China and has a total population of 108.1 million across 18 cities, including the 2 largest in western China (Chengdu and Chongqing). As most air quality monitoring stations are located in urban areas, we simulated the PM2.5 (i.e., particulate matter with an aerodynamic diameter < 2.5 µm) and ozone (O3) in the entire SCB during winter (December 2014–February 2015) and summer (June–August 2015) by using the Weather Research and Forecasting (WRF) and the Community Multi-scale Air Quality (CMAQ) models. The simulated concentrations of 24-h PM2.5 and its major components generally agree with observations during both seasons, but the simulated 1-h and 8-h O3 are acceptable only for summer. Increasing in severity from the rim of the SCB to its inner areas, the PM2.5, as well as its major components, exhibits hotspots near the central urban areas of Chongqing and Chengdu, with concentrations of 150–200 µg m–3 and 40–60 µg m–3 during winter and summer, respectively. The 1-h and 8-h O3 exhibit no hotspots in the urban centers of Chongqing and Chengdu but show elevated levels in some rural and suburban areas (55–70 ppb and 65–80 ppb, respectively), including those on the western and southwestern rim of the SCB, and downwind of the urban center of Chongqing. Despite the great spatial variations in the PM2.5 and O3 concentrations, the vast majority of the basin fails to meet the WHO guidelines for 24-h PM2.5 (25 µg m–3) and 8-h O3 (~47 ppb) on > 70% of the days during winter and > 40% of the days during summer, respectively. Based on the aforementioned spatial patterns of the PM2.5 and O3 concentrations, and the wind directions within the basin, strictly controlling emissions originating in the SCB may greatly reduce PM2.5 and O3 concentrations within the basin.