Abstract: Based on the wind tunnel experiments on the aerodynamic characteristics of the combined wing in ground effect and the towing tank experimental results of the wingtip vortex PIV velocity field, the basic rules of the difference in lift and drag between the compound wing and the straight wing were summarized, and the wingtip vortex was analyzed. By analyzing the evolution of the tip vortex flow, the variation mechanism of the aerodynamics of the compound wing in ground effect was discussed from the view of wing span-wise flow component. The compound wing is composed of a straight middle section and a rectangular wing with anhedral angle on the outer sections on both sides. Generally, the negative-dihedral in free space reduces the lateral stability of the wing and improves the roll agility, but has little effect on the aerodynamic performance of the wing. However, the results of this experiment show that under ground effect conditions, the anhedral changes the lift and drag of the wing obviously. The closer to the ground, the more obvious the lift increase effect of the compound wing with large anhedral angle. And the results of PIV measurement show that the anhedral structure of the compound wing can weaken and inhibit the generation of wing tip vortex, which is beneficial to reducing the wing induced drag. In addition, under certain working conditions, it is found that the secondary vortex occurs in the wingtip vortex field, and dissipates the strength of the wingtip vortex, thereby weakening the downwash motion and further reducing the induced drag of the wing.