Three numerical models of an abdominal aortic aneurysm (AAA) with different geometric parameters are established in order to examine the coupled effect of the fluid-structure of AAA. The study is focused on examining the roles of the dilatation parameter and the aspect ratio of an AAA in the flow dynamic within a pulse period. The numerical results demonstrate that the vortex dominates the dynamic flow behavior within an AAA. During a pressure impulse cycle, an AAA is influenced by the entirety of the vortex, from its generation to its subsequent disappearance. As an indirect effect of the vortex dynamic acting on vessels, a series of alternate impulse responses of the wall shear stress (WSS) is generated in an AAA as eddies induced by the vortex move along the axis of the vessels, and the strength of the responses is proportional to the strength of the eddies. The alternated impulse of WSS may be one of the main influencing factors on AAA rupture. The results of this study aided in understanding the mechanisms underlying the evolutionary processes of AAAs.