Abstract: In order to study the characteristics of ambient air quality in the main urban area of Yuxi city, the data of three state-controlled automatic ambient air quality monitoring stations in 2021 were selected to analyze the variation trend, pollution characteristics and influencing factors of air quality. The results showed that in 2021,the ambient air quality in Yuxi city was generally good, the average annual concentration of all pollutants met the second-level requirements of GB 3095-2012, the proportion of good days reached 99.2%, and there was no moderate or above pollution events. The main pollutant was O₃, accounting for 74%, followed by PM₁₀ and PM_2.5.From January to April and December, the ambient air quality was relatively poor, and the pollution in winter and spring was obviously higher than that in summer and autumn. Under the comprehensive influence of biomass combustion in south-east Asian countries in spring, combining with the production, living and meteorological factors, the composite pollution situation with ozone as the main and particulate matter as the auxiliary has emerged.