Abstract: Clear air turbulence (CAT) is one of the important severe weather conditions that affect the safe operation of aircraft and the travel comfort of passengers. To investigate the mechanism of a severe CAT event that occurred over Changzhi, Shanxi Province at 0210 UTC on 6 June 2017, the numerical simulation and mechanism study of this event were conducted by using conventional observation data, NOAA-15 and NOAA-18 satellite radiation data, and by meteorological analysis, mesoscale WRF numerical model and data assimilation by GSI assimilation system. The results show that the aircraft turbulence event is a typical CAT caused by the upper-level front-jet system, and the upper-level jet stream is one of the main factors causing the clear air turbulence. The response of a series of atmospheric elements to the cold air intrusion is the cause of the CAT event. The southward development of upper-level cold clouds (cold air) is an important incentive to promote the occurrence of this severe turbulence event. Due to the southward movement of upper-level cold clouds, the meridional gradient of temperature over Changzhi increases, while the meridional gradient of temperature over Changzhi increases, resulting in the increase of mid-latitude wind shear. The increased vertical wind shear results in the decrease of Ri value, and the low Ri value is the necessary condition for Kelvin-Helmholtz instability, which leads to CAT over Changzhi. In addition, the mesoscale WRF model and the GSI assimilation system can well reproduceSthe characteristics of the atmospheric cloud field near the turbulence area, indicating that the numerical prediction process in this paper has a certain ability to forecast atmospheric cloud field during the development of clear air turbulence.