The invention discloses a repair material for preventing cable water trees from aging and a preparation method and application thereof. By using the hydrogen bond effect and conjugation effect betweenmethyl, phenyl and methoxy functional groups in a composite cross-linking agent, the complex silicon cross-linking agent can penetrate into a cross-linked polyethylene cable more easily, and chemicalbonds are formed between the nano particles and the cable to fill the micro pores of a water tree and remove the moisture of an electrical insulating layer, so as to repair the aging defects of the cable. With high hardness, easy migration, good catalytic activity and surface coating ability, good mechanical and thermodynamic strength, the ability to resist wear, corrosion and cracks and good chemical stability, nano alumina can better fill the micro pores of the water tree, has larger specific surface area and is more uniform in coverage. Ferrocene absorbs ultraviolet light, prevents partialdischarge, stabilizes the voltage and improves the electrical breakdown potential of cable insulation. Sodium dodecyl sulfate disperses the nano alumina particles on the surfaces of the solution andpolymer to prevent the nano alumina particles from agglomerating on the surface of the polymer.