An important effect of the interfacial instability occurring at the interfaces of gases is to enhance the mixing of gases. In the present paper, the vortex/wall interactions at the late stage of the evolution of V shaped air/interface accelerated by weak shock wave in a duct is numerically simulated using high-resolution finite volume method with minimized dispersion and controllable dissipation (MDCD) scheme. The objective of the present paper is to study the mechanism of mixing enhancement due to the vortex/wall interactions. Because of the shock impingement, the Richtmyer-Meshkov instability is first developed. As a result, the baroclinic vorticity is deposited near the interface due to the misalignment of the density and pressure gradient right after the interaction of shock wave with V shaped interface, leading to the formation of vortical structures along the interface manifested by the Kelvin-Helmholtz instability. The vortices induce the rolling up and deformation of interface, and multi-scale vortical structures are generated because of the interaction and merging between vortices. This process eventually causes the turbulence mixing transition. The vortex induced velocity field drives the vortices to move to the lower/upper walls of the duct, leading to the complicated interaction between vortex and wall. It is observed in the numerical results that during the vortex/wall interaction, vortex is accelerated along the wall, leading to the stretching of material interface. Then the primary vortex will lift off from the wall and forms a second vortex. These two phenomena are the two main mechanisms of the mixing enhancement. Because of the inherent instability at the interface, the stretching of the interface will spread the area of instability. Furthermore, at the late stage of the interfacial instability, the flow near the interface is turbulent because of the rolling and pairing of the vortices. Therefore, the stretching of the interface will speed up the development of the interfacial turbulence and enhance the mixing. The vortex lifting off from the wall can directly speed up the mixing since it makes the heavy gas move directly into the light gas. To further determine which mechanism is dominant, we study the evolution of the mixing parameter derived from a fictitious fast chemical reaction model. It is shown that during the acceleration of the vortices along the wall and the stretching of the interface, the slope of the mixing parameter increases by a factor of 2, which indicates a significant mixing enhancement. And the vortices lifting off from the wall also shows considerable mixing enhancement but it is not so strong as the first mechanism.