The invention discloses modified fly ash for plastic filling reinforcement and a preparation method of the modified fly ash. The modified fly ash comprises the following raw materials by weight: 100 parts of fly ash, 5 to 30 parts of a thermosetting resin solution, and 5 to 20 parts of an interface reinforcing agent emulsion. The preparation method comprises the following steps: modifying the fly ash according to a twice coating modification method, that is, coating the surface of the fly ash with the thermosetting resin solution, and then coating the surface of the fly ash coated with the thermosetting resin solution with the interface reinforcing agent emulsion, so as to obtain the modified fly ash. The modified fly ash has the advantages that the surface of the fly ash is modified uniformly; an interface layer of high mutual acting force is formed between fly ash and a plastic base body, so that the bonding is firmer, the interface acting force transmission is uniform, the filling quantity in a plastic product can be increased, and even the mechanical property of the plastic product can be improved; moreover, as neither expensive coupling agent nor strong acid/strong alkali of extremely high corrosivity is adopted, the cost is low, and the operation is safe; a sealed pipeline is adopted for raw material conveying, so that the work environment is improved, and environmental pollution is reduced.