Diagnostic analysis on a server icing case occurring on the route from Luoyang to Guanghan on February 16,2006 is conducted.It shows that the severe aircraft icing has close relations to the convective instability meteorological conditions between 3000 and 7000 high above the sea level in the east of the Tibetan Plateau.Such physical variables as K in-dex,upward velocity vorticity advection,etc.can be used for the reference of icing forecast. Larger K index or its increasing areas,strong upward flow and advection regions of positive vorticity all correspond to the severe aircraft icing.