Abstract: We analyzed the pollution characteristics and meteorological causes of a heavy air pollution event in the southern Sichuan urban agglomeration (including Yibin, Zigong, Luzhou, Neijiang, and Leshan) from December 26, 2016, to January 11, 2017, based on the observational data of air pollutants' concentration and meteorological parameters and using methods of statistical analysis and weather diagnosis.The results indicated that the southern branch trough is the major weather system that contributes to the formation of this heavy pollution process.It strengthens and guides the dry, warm southwesterly flows in front of the trough, forming a local low-level isothermal or inversion layer and suppressing the dispersion of local air pollutants.The convergence of wind at low levels, the hygroscopic growth effect due to high relative humidity (without precipitation), and low-level isothermal temperature or temperature inversion, contributes to the peak concentration of air pollutants (AQI=286) in southern Sichuan agglomeration.As the south branch trough weakens, temperature inversion and low-level wind convergence disappear.The dispersion of air pollutants enhances, and air pollutants' concentration decreases.Finally, the wet removal effect of precipitation reduces pollutants' concentration quickly, ending this air pollution event.This study provides new important reference for understanding the meteorological causes of heavy pollution in the southern Sichuan urban agglomeration and its prevention and control, which is different from the causes for air pollution in the northwestern Sichuan Basin.