The invention relates to the field of water resources and hydropower engineering, and especially relates to a high-speed flow open-flow drain tunnel air compensation structure building method and an air compensation structure, and the method provides convenience for design personnel to conveniently and efficiently design a drain tunnel air compensation structure and construct the drain tunnel air compensation structure. The method comprises the following steps: a, firstly, determining a structure of an open-flow drain tunnel, according to the structure of the open-flow drain tunnel, determining flow velocity variation and positions which needs high air compensating amount; b, from an underground working lock chamber in an open-flow drain tunnel, arranging totally n tunnel spillway air compensation holes in a segmented manner in sequence; c, according to sectional area of each tunnel spillway air compensation hole obtained in the above step, determining construction area size of an end face of the tunnel spillway air compensation hole; d, according to the determined positions and sizes of the open-flow drain tunnel and the tunnel spillway air compensation holes, performing construction, to finally complete the high-speed flow open-flow drain tunnel air compensation structure. The method is especially suitable to be used in design and construction of various kinds of high-speed flow open-flow drain tunnel air compensation structures.