In order to solve the difficulties of the multi-color, wide wavelength band, long working distance and high numerical aperture in imaging flow cytometer, an optical system of multispectral achromatic imaging flow cytometer is designed. Based on the modular design concept, the relative independence of each module is guaranteed by the flexible use of the local and whole optimization method. The microscope objective introduces diffractive optical element to correct chromatic aberration in the wide spectrum. So the imaging quality can meets ideal performance in full-wave band. The multispectral group uses dichroscope stack to split light, and six channels are optimized by multiple structure. The difficulty of chromatic aberration correction is reduced greatly. The final optical system is got by global optimization. The magnification is 60, the field of view is 60 μm×128 μm, the wavelength range is from 420 nm to 800 nm, the system has six channels, the resolution is 0.5 μm, and the imaging quality is close to diffraction limit.