In order to explore the impact of large-scale circulations on O₃ pollution in Chengdu, based on the 700 hPa geopotential height field in the Sichuan Basin (SCB) and using the T-PCA method, the atmospheric circulations in the summer of 2015–2019 (June–August) were classified into five circulation patterns, and the characteristics of O₃ pollution and O₃ sources in Chengdu under different circulation patterns were identified. Among the five types of circulation patterns, the northeast-high pressure (NEH) pattern and the under-high pressure field (UNF) pattern generally led to higher air temperature, and lower humidity and cloud cover in the SCB. Air temperature change compared to the previous days showed an obvious increasing trend in the SCB under the UNF pattern, kept stable under the NEH pattern, and indicated cooling effects with varying intensities under the other three circulation patterns (the southeast-high pressure pattern, the northwest-low pressure pattern and the northwest-high pressure pattern). The meteorological conditions under NEH and UNF were more conducive to the photochemical production of O₃ in the SCB. During the study period, O₃ concentrations and the percentages of over-standard days of O₃ in Chengdu under NEH and UNF were significantly higher than those under the other three circulation patterns, and thus NEH and UNF were the main circulation patterns that were related to O₃ pollution in summer. Months with more severe O₃ pollution in Chengdu tended to have higher proportion of days under the NEH and UNF patterns. The simulation studies of O₃ sources in Chengdu in summer suggested that O₃ was mainly derived from the SCB (55.0%), with local contribution of Chengdu (31.6%), and other important contributing cities including Deyang (5.4%), Chongqing (4.0%), Ziyang (3.9%) and Meishan (2.2%). However, O₃ sources in Chengdu under different circulation patterns showed different characteristics. Under the NEH pattern, O₃ transport within the Chengdu Plain had notable impact on O₃ pollution in Chengdu, with a significant increasing contribution from Deyang (14.9%). By contrast, under the UNF pattern, O₃ pollution in Chengdu became more localized, with nearly half (46.8%) of O₃ contributed by local emissions.