Abstract: CAT (Clear-Air Turbulence) is one of the important severe weather conditions that affect aircraft safety and passenger comfort. The mechanism of a severe CAT event that occurred over Changzhi, Shanxi Province, at 0210 UTC on June 6, 2017, was investigated via numerical simulation and a mechanism study of this event using conventional observation data, NOAA-15 and NOAA-18 satellite radiation data, meteorological analysis, a mesoscale WRF (Weather Research and Forecasting) numerical model, and a GSI (Grid point Statistical Interpolation) data assimilation system. Results show that the aircraft turbulence is caused by the CAT, primarily resulting from a typical upper-level front-jet system. CAT is triggered by a series of atmospheric responses to the cold air intrusion. The southward movement of upper-level cold clouds (cold air) mainly promotes this severe turbulence event and increases the meridional gradient of temperature over Changzhi, thereby increasing the mid-latitude wind shear. The increased vertical shear of horizontal wind decreases the Ri (Richardson number) value, which causes K-H (Kelvin-Helmholtz) instability and leads to CAT over Changzhi. In addition, the mesoscale WRF model and GSI assimilation system are capable of simulating the characteristics of the atmospheric temperature field near the turbulence area with good accuracy. This indicates that the numerical prediction process used in this study possesses certain forecasting capabilities for atmospheric temperatures, wind fields, and CAT development.