The invention discloses a preparation method of composite plastic for a power supply bracket, which comprises the following steps: mixing expanded graphite and an ethanol water solution, carrying out35-45kHz ultrasonic dispersion for 20-30 minutes, conducting stirring at 80-90 DEG C under a reflux state for 1-2 hours, and conducting cooling to 40-50 DEG C to obtain pretreated expanded graphite; stirring the pretreated expanded graphite, slowly dropwise adding tetraethoxysilane in a stirring state, continuously conducting stirring after the tetraethoxysilane is completely dropwise added, conducting cooling to room temperature, performing suction filtration, conducting washing to be neutral, conducting dispersing in an ethanol aqueous solution, and performing spray drying to obtain a silicon dioxide-expanded graphite compound; and uniformly mixing polypropylene, a styrene thermoplastic elastomer, the silicon dioxide-expanded graphite compound, fumed silica, wollastonite powder, ceramicfibers, talcum powder, maleic anhydride grafted polypropylene, far infrared ceramic powder, lignocellulose, a coupling agent, a flexibilizer, a lubricant and an antioxidant, carrying out melt extrusion, cooling, and granulating with a granulator, and conducting drying to obtain the composite plastic for the power supply bracket.