Based on numreical study and analysis of the continuous torrential rainstorm in Northwest Fujian during 14～18 June,2002,it is pointed out that enhancing of high-level jet stream and spreading downward of momentum inpelled the establishing and maintain of low-level jet stream under the advantage of the circulation background .The enhancing of low-level jet stream provided the megathermal and high wet condition of atmosphere which is beneficial for causing heavy rain.The proper place of the high and low level jet stream produced the function of coincidence on dynamical field and provided continuous dynamical condition with which torrential rainstorm occurred and developed.