Abstract: Based on the tendency equation of turbulent kinetic energy, its simplified equation including only wind shear and air stability was derived. In addition, this simplified equation was tested using the environmental technical application (ETA) mode and the data of 100 cases of clear air turbulence (CAT) in flight. The research shows that the prediction of CAT in flight should not be based on only the generation of turbulent kinetic energy, and it is necessary to diagnose the generation item for turbulent kinetic energy. The dissipation item in the tendency equation of turbulent kinetic energy can not be neglected in the prediction of CAT, and the vertical divisional rate of the ETA mode and the air stability given by the mode designer have also an effect on the precision of CAT prediction.