Combined with the case of expressway widening and reconstruction project, based on the finite element calculation technology, the mechanical response analysis of the existing asphalt pavement structure of expressway under different traffic conditions was carried out. According to the calculation results the differential pavement structure design scheme with different material properties on the upper and middle surface layers was proposed. The shear stress of the vertical interface in the upper and middle surface layer and the vertical surface of the lower layer formed between the adjacent light-load and heavy-load lanes within the design period are analyzed. And the influence law of the contact state and the position of load on the shear stress and the possibility of shear failure are studied. The results show that the shear stress distribution law of interface and lower vertical surface is different when the load position is different. When the load is located in the center of the heavy-load roadway, the shear stress increases gradually with the increase of pavement depth, and the maximum value appears at the bottom of the semi-rigid material base layer. However, at the top of the interface, the shear stress gradually decreases with the increase of pavement depth, and the maximum value appears on the pavement surface, which can produce a large shear stress leading to shear failure. However, the probability of the load continuing to act on the top of the interface is small, so the shear failure at the interface will not actually occur, but the shear failure at the track zone is almost inevitable.