Abstract: A cold air outbreak with simple synoptical process is selected as an example for this investigation. The field of the upper horizontal temperature and the structure of the cold front are discussed.As the front moves from north to south over China, the slope of the front in the lower level steepens and its intensity increases.In the late stage of the cold air outbreak,the upper horizontal temperature field changes rapidly. Computation by frontogenetic formula shows that the term of differential temperature advection is predominant.