Abstract:PM2.5 and O3 which interact with each other have become the most important pollutants in the cities of Fenwei Plain. The pollution often occurs simultaneously in warm seasons and is closely related to meteorological conditions. Based on the daily PM2.5 and O3 concentration data, surface meteorological observation data and ERA5 high-altitude reanalysis data of 12 cities in the Fenwei Plain from 2015 to 2021, the spatio-temporal characteristics of PM2.5 and O3 in the Fenwei Plain, as well as the relationship between PM2.5 and O3 when compound pollution occurs are analyzed. Besides, the impact of local meteorological conditions and synoptic situation on compound pollution is also studied. The results show that the average annual PM2.5 and MDA8 O3 (maximum daily 8 h average ozone) concentrations in the Fenwei Plain began to decline continuously in 2017 and 2018, respectively, and the number of compound pollution days also began to decrease continuously after 2019. Compound pollution mainly occurs in March to September, frequently seen in the cities of eastern Fenwei Plain, mostly in condition of high temperature and low humidity. Finally, the synoptic circulation situation of compound pollution is divided into four types by principal component analysis with the T-PCA algorithm, featured with the northwest or westerly air flow in high-altitude and the southerly wind or breeze in the low-level warm zone. These findings could provide some basis for controlling pollution in the Fenwei Plain and are of great significance for cooperative governance of PM2.5 and O3 pollution.