The present invention relates to an ultra-thin acoustic lens for subwavelength focusing in a megasonic range and a design method thereof. More particularly, the present invention relates to a super-oscillatory planar ultra-thin acoustic lens for subwavelength focusing in the megasonic range, which includes a plurality of concentric regions arranged in a concentric shape with reference to the center point, wherein the concentric regions include a plurality acoustic insulation region for insulating incident acoustic waves, and a plurality of transmission regions for transmitting acoustic waves, the acoustic insulation regions and the transmission regions being formed alternatively in a radial direction from the center point so as to focus incident acoustic wave energy onto a subwavelength region. The acoustic lens has flat surfaces on both sides thereof respectively and has a plate shape having a constant thickness, and a layout, which is a radius of each of the plurality of acoustic insulation regions and transmission regions in the concentric region, is determined by a topology optimization reverse design method.