Abstract: The stepwise approximation method was applied to analyze sounding data,ground data,AQI data and the six main pollutants(SO₂,NO₂,CO,O₃,PM2.5,PM₁₀) mass concentrations data collected at Wenjiang Station,Yibin Station,Dachuan Station and Shapingba Station in Sichuan Basin during 2015-2018,and calculate daily maximum mixing depth(MMD). At the same time,the time variation characteristics of MMD and the relationship between the MMD and the concentration of various pollutants were analyzed. The results show that the annual average MMD of Sichuan Basin is about 1200m. The seasonal variation of MMD is obvious,larger in spring and summer but smaller in autumn and winter (relatively small from September to January of the following year). Correlation analysis showed that after excluding the effect of precipitation,MMD was negatively correlated with AQI,as well as the mass concentration of PM2.5,PM₁₀,SO₂,NO₂,CO,but positively correlated with the mass concentration of O₃. Pollution is the most serious in winter,and the MMD in winter is significantly smaller than that in spring and summer. The smaller the MMD,the higher the concentration of particulate matter. Small MMD greatly compresses the diffusion space of near-surface pollutants,which accumulate and increase in concentration in a limited space.