Abstract: In order to investigate the causes of heavy pollution in Tianjin during the epidemic period of COVID-19 in 2020, the environmental monitoring, routine meteorological observation and 255 m meteorological tower gradient observation, together with WRF-Chem model simulation were used to discuss the sources and boundary layer characteristics of heavy pollution process during February 9-13th, 2020 in Tianjin. The results showed that the deterioration of horizontal and vertical diffusion conditions, weak air pressure field and warm and humid atmosphere on the ground were the main weather characteristics of this heavy pollution case. The regional transportation rate of external sources in the process of heavy pollution was 54.6%. The stable stratification (E and F) accounted for 67.5%, and the relatively stable atmosphere decreased the atmospheric diffusion condition, which was an important meteorological condition for the occurrence of this pollution. The inversion rate of the pollution process reached 50.0%, and the correlations between the vertical temperature difference, inversion thickness, inversion intensity and PM_2.5 concentration were 0.99, 0.90 and 0.56, respectively. The existence of inversion layer was the main factor to maintain the pollution process.