Abstract: A computational and particle image velocimetry(PIV) experimental study of three dimensional steady separation structure generated in juncture laminar flows was presented. It confirms the existence of outmost attachment saddle point structure of horseshoe vortex in juncture flows. The outmost streamline doesn’t rise from the plate but attaches to the plate by passing through a space singular point. There also exists an evolvement rule of horseshoe vortex structure from outmost attachment saddle point structure to outmost separation saddle point structure. The evolvement is influenced by the blunt extent of the model, the boundary layer thickness and the Reynolds number. Under curtain boundary layer and the Reynolds number conditions, the horseshoe vortex will change from outmost separation saddle point structure to outmost attachment saddle point structure as the blunt extent of the model becomes smaller. For a same model, the horseshoe vortex will tend to be outmost separation saddle point structure as the boundary layer thickness and the Reynolds number increases, contrarily it will tend to be outmost separation saddle point structure.