The invention discloses a preparation method of a planarized coating layer for a diamond with an ultra-large-aperture inner hole. Direct-current forward deviation deposition, direct-current reverse deviation deposition, direct-current reduction deposition and optimization deposition are carried out on the surface of the inner hole of a drawing die in sequence by adopting a heater chemical vaporous deposition method so that the planarized coating layer of the diamond with the ultra-large-aperture inner hole is obtained. In the preparation method, a three-phase rectifying direct-current power supply is used instead of a single-phase alternating-current power supply so that the problem of three-phase balance in high-power supply is solved; and meanwhile, not only is the quality of the diamond coating ensured, but also the surface smoothness of the coating is improved, the polishing workload of the coating is reduced, and the qualification rate and the reliability of a coating die are improved.