It is suggested that uncontinuous propagation of cold front is the direct cause of fast -southerly movement of cold fronts,i. e. ,as a result of frontogenesis in the southern part of the cold front and its evolution,the new frontogenetical front takes the place of the cold front in north China. The southwesterly low-level jet plays important roles in the frontogenesis process,these roles are: (l)northerly warm-wet transportations in the low-level jets;and (2) the jets can induce low-vortices in the north of jets,and northerly air current of west of the vortices is favourable to south ward movement of cold air mass. The result of interaction of the air currents which possess distinct property induces frontogenesis. The caues of the low-level jets formation in winter arc also preliminarily discussed. It shows that they are different from those in the summer.