In this paper,the mumerical simulation of precipitation during the period of 0000Z 5th to 1200Z 6th July 1991 is carried out by using the MM5 meso-scale model.Through the diagnostic analysis of simulated 3-hour interval results,it is found that the trend of upper level jet was close related to the motion of southwest vortex.When the northwest,west and southwest jet lay over the east of China,the southwest vortex was stationary,moved eastwards rapidly,and strengthened and moved slowly,respectively.Meanwhile,the motion of rain core was consistent with the eastward motion of the southeast vortex.At first,the rain belt was stationary,then it moved quickly eastwards,and at last,its movement became slow.Wet potential vorticity had a great contribution to the stronger baroclinicity of the southwest vortex.Before the baroclinicity of southwest vortex had not been built and in areas near the cold front,the rain belt might lie in anywhere of the left of southwest low level jet,but when the baroclinicity of low level vortex was strengthened and when the warm air came into being,the rain belts all occurred in the left front of southeast low level jet.