The gas flow of cyclones with various vortex finders was numerically simulated using the commercial computational fluid dynamics（CFD）software package，FLUENT 6.1.The result indicated that，two types of axial velocity structures appeared，upended V-shape and M-shape，by varying the diameter of vortex finder.With the increase of vortex finder diameter，the distribution pattern of axial velocity changed from upended V-shape to M-shape.The dip of axial velocity appeared inside the vortex finder initially and then grew up to the separation space continuously.When the vortex finder diameter was larger than a certain value，the back-flow could even extend to the whole inner separation space of cyclone.Likewise，the radial region of back-flow increased.While，the axial velocity nearby the centerline decreased with the increasing vortex finder diameters，and became negative even.In addition，the structure of vortex finder，flared end or shrink end，also has some effect on the axial velocity.It was considered that the distribution pattern of axial velocity was related to the axial gradient of pressure.When the energy loss in the vortex finder was small to a certain value，the pressure gradient turn to positive，and the back-flow of axial velocity appeared.