Abstract: Based on the data of environmental monitoring, conventional meteorological observation and NCEP reanalysis, aerosol lidar and HYSPLIT model were used to analyze one PM_2.5 heavy pollution process in Tianjin in summer on Aug.1 and Aug.2, 2018. The results showed that the weak pressure of surface field, temperature-inversion layer in the boundary and east warm-humid airflow accelerated the accumulation of pollutants. Aerosol lidar analysis showed that the pollution process had obvious horizontal and vertical distribution characteristics. The increasing of PM_2.5 concentration in urban area was not only related to the horizontal transport, but also closely related to the accumulation of pollutants caused by temperature-inversion layer in the boundary. The backward trajectory tracking by HYSPLIT model showed that in the early stage of cumulative climb of PM_2.5, air masses mainly came from south, and the air masses had obvious settlement at 200, 500 and 1 000 m altitudes during the climbing phase of PM_2.5 concentration. The eastward warm airflow brought the clearer air as well as a lot of moisture, which resulted the increasing of relative humidity during the later stage pollution. The static weather situation caused the accumulation of pollutants on the early stage, while and the transportation of pollutants between the urban areas of Tianjin as well as the increasing of relative humidity caused by the eastward warm humid airflow aggravated pollution on the later stage.