By using the NCEP reanalysis data, conventional ground and radiosonde observation, satellite and Doppler radar data, etc., the causes why the storm and ground wind strengthen in the central North China are analyzed in detail. The results show: (1) On 5 August 2017, under the influence of a cold vortex, there was dry air infiltration in the middle and upper levels and conditional instability in North China; at 11:00, according to the revised radiosonde data in Tianjin, the CAPE reached 3184 J kg-1 and the lower level water vapor was abundant, which contributed to the occurrence of thunderstorm gale and moist convection. (2) The merger of the storm outflow boundary and the ground convergence line in the central North China and the higher surface dewpoint temperature in the southeastern North China are important reasons for the strong development of the squall line system in central and southern China. (3) The high temperature and humidity environment made the storm spread southward, and under the guidance of the northwest airflow, the storm finally moved south by east. (4) At 08:00, according to Beijing radiosonde data, the vertical wind shear of 0 to 6 km reached 3.3 m s-1 km-1; the air flow rose in the front and sank in the back. Strong vertical motion and vertical wind shear produced strong rotation, which made the squall line system have the characteristics of mesocyclone in the early stage. (5) Strong convergence in the middle layer and divergence at the storm top produced violent downdrafts. The maximum wind on the ground occurred in the development stage of mesocyclones and the merging of cold pools.