Abstract: A diagnostic study has been made on the ageostrophic wind and the primary factors which can affect the ageostrophic wind in strong cold air outbreak in winter over East Asia. The results has indicated that the ageostrophic wind is one of the most significant factors. It may cause the strengthening of the surface and lower level north wind. Agostrophic secondary circulations are formed near the frontal zones as well as in the entrance and exit regions of the upper level westerly jet, resulting in the strengthening of the jet stream and local Hadley circulation in East Asia. However, the upper level westerly jet in East Asia is subgeostrophic. The allohypsic wind, inertial advection and friction effect are the main factors which influence on the ageostrophic wind. In addition, the local change of the potential temperature, the advection of the potential temperature and diabatic heating play a certain role in the formation of the ageostrophic wind, but the composite wind vector of these three ageostrophic wind components is very small due to the offsetting of them. The ageostrophic wind plays a very important part in the kinetic energy budget in limited areas in East Asia.