Abstract: Evolution mechanism for wake-type multi-vortex flow over a slender body is studied using numerical simulation. At lower incidences, the symmetric flow pattern of three level vortices contains the origin of the new main vortex in the asymmetrical multi-vortex series. The dynamical process, which accompanies the spanwise movement of vortex series and vorticity transferring, is described clearly for main vortex shedding following tertiary vortex developed to a new main vortex and merged into the secondary vortex on the opposite side of the symmetry plane. Since the main vortex is generated and shed right-and-left, a three dimensional vortex series is developed at the leeside of the slender body. Though the sectional flow pattern of the three dimensional vortex series is very similar to one of the Karman vortex, evolution mechanism of these two vortex series is different.