This study analyzes the characteristics of water vapor transport and precipitation structure of an extreme precipitation event occurred over the Beijing-Tianjin-Hebei (BTH) region on 12 July， 2021， based on the ERA5 reanalysis data， Himawari-8 geostationary satellite cloud property data， precipitation structure data from the dual-frequency radar of the Global Precipitation Measurement (GPM) mission， and precipitation amount data from the Integrated Multi-satellite Retrievals for GPM (GPM IMERG). The southwesterly airflow on the western edge of the western Pacific subtropical high transport moisture from low-latitudes to northern China. An Atmospheric River (AR) is formed due to obstruction by a low vortex over the Yellow River Basin and a high-pressure ridge over the Bohai Sea. Inside the AR， a significant moisture convergence belt is seen over the BTH region. It provides ample water vapor condition for heavy rainfall. The vertical profile analysis shows that during heavy rainfall period， strong upward motions penetrate almost the entire troposphere， overlapping with the region of strong moisture convergence. On the morning of 12 July， areas of heavy rainfall coincide with strong moisture convergence regions generally. In the afternoon， heavy rainfall occurs also in the regions of moderate convergence or weak divergence. The cloud top heights in heavy rainfall regions generally exceeds 12 km. The high cloud top area roughly overlaps with the strong precipitation area. This extreme event is dominated by convective precipitation. Compared with climatology， radar reflectivity factors display dual high-frequency peaks of 20 dBZ and 30-34 dBZ at lower levels. Hydrometeors are concentrated at lower levels between 1-2 km， characterized by smaller diameters and higher number concentrations. Additionally， particle diameter profiles are notably smaller at altitudes between 6 and 10 km， showing irregular variations with precipitation intensity but higher number concentrations.