Abstract: The interactions between middle and low latitude synoptic system in the winter season of the Northern Hemisphere and their relationship with precipitation in the south of China have been comprehensively studied. The results show that when precipitation occurs, the polar-front jet is merged with subtropical jet, and then intensified, meanwhile, the jet entrane appears supergeostrophic wind. This phenomenon sometimes is connected with the increase of Hadley circulation and the convection caused by cold surges of the Northern Hemisphere arriving middle and low latitude, and with its feedback. It offers favorable conditions for precipitation in the south of China.