Abstract: Two dimensional wind tunnel tests of multi-element airfoil are simulated by using Reynolds averaged Navier-Stokes equations. Skin friction lines on side wall, streamlines and total pressure contours at different cross sections are combined together to depict the topology of vortex flow of multi-element airfoil. And the side-wall interference on the merging process of vortex pairs is studied. The wall confined merging process of vortex pair is evidently different compared with the unconfined merging process. The formation of focus from shear layers, the development process from focus to center (vortices) by absorbing shear layers, are both slowed down by the intrusion of sidewall. Side wall also dissipates the vorticity and decreases the kinetic energy of vortices by skin friction, but its interference is limited to certain zones. According to the numerical results, it is suggested that, the test section of wind tunnel must be wide enough for two dimensional tests; for semi-span model tests of high aspect ratio airplane, stand-off method or fuselage image can be employed. Full model test is necessary for small aspect ratio model.