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We propose two kinds of high-efficiency free-space wave splitters in the mid-IR band using reverse design. The wavelength divider based on the abnormal reflection principle realizes a beam-splitting angle of 22.00° and 10.92° by controlling the phase distribution, and the reflection efficiency of both wavelengths exceeds 50%. The wavelength divider designed based on the concept of metalens simultaneously accomplishes the functions of focusing and beam splitting. It has a focal length of 100 μ m and a relative focal position of 100 μ m. Most importantly, the focusing efficiency for the two wavelengths reaches an impressive 48.59% and 72.51%, respectively.