Abstract: Underexpanded impinging jets have complex shock structures and generate high-amplitude tones with discrete frequencies. Based on the schlieren images obtained by the high-speed camera and the results of noise measurements, the oscillations of shocks, the modes of instability waves in the jet shear layer and the generation of the discrete tones of underexpanded impinging jets were studied. In this paper, the experimental results of the impinging jet with an impact distance of 5 nozzle exit diameters were presented. It is found that there are two modes of the instability waves in the shear layer: symmetric and asymmetric modes. The sound source of the impinging tones at different modes is located at the instability flow structures after impinging on the plate. The feedback acoustic waves excite the shear layer at the nozzle lip and maintain the development of the instability waves. The oscillations of shocks are caused by the interactions between the shocks and the shear layer and are related to the characteristics of the instability modes. By weakening the disturbances of the feedback acoustic waves on the shear layer at the nozzle lip, the discrete tones are eliminated and the shock oscillations are suppressed.