Abstract: To solve the problem of beam splitting limitation of cemented cubic beam splitters in the mid-wave infrared band, the optical design scheme of mid-wave infrared Fizeau interferometers based on wedge splitting is proposed. At the working wavelength of 3.39 μm, to reduce the return error of the interference system and improve the measurement accuracy, a two-reflection folding collimating optical path structure is adopted, which not only ensures a good collimating wavefront, but also optimizes the design of the optical wedge to take into account the wavefront quality of the interference imaging. ZnSe and CaF₂ materials are used, the collimator of the interferometer is a single plano-convex aspheric structure, and the imaging lens is composed of two separate spherical mirrors. Through the Montecarlo simulation tolerance analysis, the collimator wavefront PV of the collimator within 0.1° field of view is better than λ/4. The normalized field of view imaging wavefront PV of the interferometric optical path is better than λ/5; The interferometric system return error is smaller than λ/50 at 0° field of view placed on the standard surface and the surface under test is tilted within 0.05°.