From the afternoon to night on 5 July 2021, severe convective weather including short-time heavy precipitation, thunderstorm wind and small hail occurred in the central region of Beijing-Tianjin-Hebei. The atmospheric environmental conditions and mesoscale characteristics of the formation of this weather process were analyzed by using the data of egional automatic meteorological station, Doppler radar, FY-2G meteorological satellite and microwave radiometer, and the fifth generation atmospheric reanalysis ERA5 from European Centre for Medium-Range Weather Forecasts. The results show that favourable water vapor conditions appeared before the occurrence of the severe convective weather including heavy precipitation, thunderstorm winds and local hail. Strong convergence of water vapor fluxes in lower and middle layers occurred 1 to 2 hours earlier than precipitation. The whole layer atmospheric precipitable water had accumulated continuously under strong uplift. The heat and power unstable environment of severe convection breaking out was created by formation of vertical θ_se energy frontal zone, maintenance of "upper dry and lower wet" unstable stratification, establishment of strong vertical wind shear from 0 to 6 km and enhancement of CAPE, K and SI indexes. Unstable stratification formed by eastward movement of the upper though carrying dry and cold air southward and low warm tongue, which had provided synoptic-scale upward movement for the occurrence of severe convection. The strong convective happening released more energy in the afternoon than in the evening. Heavy rainfall caused the local temperature to drop significantly and cold pool effect was more significant, which corresponded to the heavy precipitation area during the southward movement process. The surface convergence line at the cold pool boundary was the mesoscale trigger system. The cloud base height dropped and infrared brightness temperature increased rapidly, which indicated the formation of strong convective cloud cluster. The clear shadow at the southeast boundary of cloud body indicated the strong development of cumulonimbus cloud. Under the background of large-scale weather system, the important characteristic indexes obtained from in-depth analysis of mesoscale system can be used for the short term forecast and warning of severe convective weather.