A dynamic analysis is made of the occurrence of a low-level jet stream with emphasis placed on the circulation structure in the jet region as well as the effect of the cumulus convection on its formation and evolution. It is found that a Couple of transversal ageostrophic secondary circulations in the low-level jet region plays a significant dynamic role. The static instability field coupling caused by the strong dispersive Iowlevle jet and the cumulus convection induces unstable mesoseale gravitational inertia waves which trigger the development of the jet stream and secondary circulation through Wave-CISK mechanism. Both the ageostrophie characteristics and the secondary circulation in the evolution of the jet are the products of the development of the gravitational inertia waves.