Abstract: In recent years the research on surface-initiated longitudinal cracking along wheelpath (or Top-Down Cracking) has become a hot topic in the field of pavement damage focused by international asphalt pavement engineering. Because the traditional load mode involves only the vertical surface load with uniform tire pressure, the load is applied at only one position and no effort is made to distinguish between tire patterns. The tradition method cannot explain the mechanism of the Top-Down Cracking (TDC). In order to discuss the mechanism of TDC, a 3D finite element model of semi-rigid pavement structure is established and the finite element software ABAQUS is used. The analysis shows that maximum shear stress appears in the surface of pavement, and is located the edge of wheel path in the form of the longitudinal cracks. Maximum shear stress is beyond the shear strength of asphalt binder. So it is thought to be the major reason leading to Top-Down Cracks. Heavy vehicles with no matter the vertical pattern tire or the transverse pattern tire will do great damage to the pavement surface. Heavy vehicles with the vertical pattern tire will cause greater damage to the pavement surface than those with the transverse pattern tire. Different forms of tire patterns, which lead to different forms of the cracks, have different influence on the pavement. The tire with the transverse pattern will be easier to generate the oblique cracks.