The preparation method of mold based on computer generated holography(CGH) of asymmetric multi-step diffractive optical element was introduced, the working principle and design method of phase-type CGH were studied, the corresponding optical system and the diffracted light wave model were established and the algorithm flow chart for obtaining phase-type mold microstructure was designed. Taking a folded heart pattern as an example, the phase information and surface microstructure morphology of 2-step, 4-step, 8-step and 16-step diffractive optical element were simulated by MATLAB, and the quality of the reconstructed image was compared on the basis of theoretical analysis. It is found that the more the number of steps is, the better the quality of the reproduced images will be. After obtaining the height data of the die space and the distribution of the surface structure, the 2-step and 4-step molds with the 6 mm×6 mm element size and the smallest characteristic size 30 um were respectively obtained by the fast knife processing method and the single-point diamond turning technology, and the step contour curve and surface structure contour of actual processing were obtained. Finally, the ultraviolet(UV)-curing nanoimprint technology was used to realize the replication process of the 4-step mold, and the image of replicated samples were produced. The results show that the method can be used for the mold preparation of asymmetric low-step diffractive optical elements.