The invention discloses a flat spring. The flat spring comprises a sheet-shaped substrate provided with a plurality of hollow areas spaced apart from each other. The hollow areas comprise three hollowunits of an annular array, wherein the three hollow units are connected with each other; each hollow unit comprises a hollow head arranged on the outer side and a hollow tail arranged on the inner side, the hollow head and the hollow tail being connected to each other; and one hollow head of one hollow area is disposed between two adjacent hollow tail parts of the other hollow area. The inventionalso provides a cushioning structure comprising the flat spring and having the functions of being flexible, breathable and easy to clean. The invention also provides a helmet, which includes the cushioning structure and has better comfort and breathability than a conventional helmet and is suitable for use in hot weather.