A design method of a dual-wavelength field-widened Michelson interferometer (FWMI) used as a spectral discriminator of a high-spectral-resolution lidar (HSRL) is proposed. The theoretical principle of the dual-wavelength FWMI is elaborated in detail. The influence of dispersion on the frequency discrimination performance is considered. The dual-wavelength field-widened design is adopted to compensate for refractive index, so that FWMI has a larger receiving field of view at 355 nm and 532 nm. The specific parameters of the dual-wavelength FWMI suitable for 355 nm and 532 nm are given, and performance evaluation and tolerance analysis are carried out. The evaluation results show that the dual-wavelength FWMI has a receiving field of view of more than 6° and excellent and stable performance at both wavelengths, and the requirements of processing precision and assembly precision are not high. Finally, Monte Carlo simulation is used to analyze the inversion error of optical parameters of the HSRL system using the dual-wavelength FWMI. The results show that the retrieval errors of aerosol backscattering coefficient and extinction coefficient are 1.82% and 11.39% at 532 nm, and 1.62% and 2.95% at 355 nm, respectively.