The invention discloses a preparation method of modified resin-based engineering plastic and belongs to the technical field of preparation of plastics. The preparation method of the modified resin-based engineering plastic, disclosed by the invention, has the benefits that polyvinyl chloride is mixed with polyamide resin to obtain mixed resin, the mixed resin is modified with antimony trioxide andred phosphorus, and then graphite powder is modified with a concentrated sulfuric acid solution, and is blended with acetic anhydride and potassium permanganate, and thus, expanded graphite powder isprepared at high temperature; the expanded graphite powder is a carbon source in an expansion system, also is a heat insulating layer and can exert an obvious heat insulating effect; steel fibers areadded in a resin base material, interlaced resin base material molecular chains are connected among the steel fibers, and the steel fibers are taken as a filler to modify engineering plastic; a layerof coconut oil is continuously smeared in an engineering plastic blank, so that the further corrosion is prevented, and the service life of the engineering plastic is prolonged; and a formula is goodin stability and has a long-lasting corrosion inhibition ability, thereby improving the corrosion resistance of the engineering plastic and having a wide application prospect.