The invention provides a preparation method for a porous implant based on additive manufacturing of a macromolecular polymer. The preparation method comprises the following steps that a macromolecular polymer negative-type mould of a porous implant model with a gradient microstructure is prepared through an additive manufacturing method; metal powder and adhesion agents are mixed evenly and pressed in the macromolecular polymer negative-type mould, then the metal powder and the macromolecular polymer negative-type mould are placed in an organic solvent for removing the macromolecular polymer negative-type mould, and a preliminary porous implant is obtained; the preliminary porous implant is placed in a vacuum high-temperature furnace, a metal coating is deposited on the surface of the implant through a chemical vapor deposition method, so that the strength of the implant is further enhanced; and finally, the metal porous implant is placed into an electrolyte to be subjected to anodic oxidation treatment, and the customized metal porous implant with a surface nanostructure is obtained. According to the preparation method, additive manufacturing and the powder metallurgy technology are combined, the problem that the pore size and pore distribution cannot be controlled is solved, nanocrystallization of the surface structure is achieved, and a new approach is opened for preparation of the porous implant with the macroscopic structure and the micro-nano structure.