The serious chromatic dispersion of traditional diffractive optical elements (DOEs) limits their application in wide-band imaging. In this paper, a design of achromatic diffractive lens is proposed. After coding the microstructure height on the whole diffractive lens, the microstructure height is optimized by the particle swarm optimization algorithm to balance the focusing effect of each wavelength in the visible spectrum at a specific focal length, so that the point spread function (PSF) of each wavelength in the visible spectrum becomes almost the same, achieving the goal of achromatism in the continuous visible light band from 400 nm to 700 nm. The simulation and imaging test of traditional DOE, zone achromatic DOE, and coding optimized achromatic DOE proposed in this paper are carried out. Compared with the other two DOEs, the PSF of coding optimized DOE at each wavelength is more consistent, and the cut-off frequency under visible light is increased to 8.6 lp/mm. Therefore, compared with traditional DOE and zone achromatic DOE, the coding optimized DOE proposed in this paper has better achromatic ability and higher resolution in visible light.