A cold-air windstorm processe (Beaufort wind force scale 10 or above) affects Shanxi, Hebei, Beijing, Tianjin, and Shandong successively from noon to night on 18 March 2020. The synoptic characteristics of the event are studied by using data of wind profile radars, national surface automatic weather stations, and ECMWF (European Centre for Medium-Range Weather Forecasts) Reanalysis v5 (ERA5). The results show that the strong gale takes place in the abnormally developed thermal environment of low-level warm ridge when the cold front intrudes into the thermal low pressure with strong frontogenesis. The frontolysis occurs on the middle level when the cold front moves downward from the Loess Plateau to the North China Plain and shears off on the middle level. The strong wind has significantly ageostrophic character and transient variation. The low-level strong cold advection is the primary factor for severe allobaric gradient. The ageostrophic allobaric wind, which is superimposed on the fast moving cold front system, causes the highest wind speed, and the allobaric wind is the essential part of the strong wind. The lower-layer downward momentum effect induces the fluctuation of wind speed, but is not powerful enough to produce the strong wind directly.