When traditional diffraction lenses work in a wide band, the results have severe dispersion and poor image quality. Thus, an optimized design method for a partition diffractive achromat based on RGB three-band is proposed. We use scalar diffraction theory to simulate and analyze the image of equal width diffractive achromats and equal area diffractive achromats which compare with traditional diffraction lenses in point spread function and diffraction efficiency. The analysis results show that the proposed partition diffractive achromat can reduce the standard deviation of the three-wavelength diffraction efficiency from 0.6607 of the traditional diffractive lens to 0.1519 and 0.0592 when working in the RGB three-band while maintaining imaging effects well. Finally, considering the actual processing conditions, the optical performance parameters corresponding to the microstructure quantization into eight steps under different partitioning methods were simulated. The results show that the proposed design method can make the diffraction lens achieve a good achromatic effect in the RGB three-band. Moreover, the proposed method has practicability and universal applicability.