The invention relates to the field of polymers, and in particular relates to a preparation method of vegetable carbon plastic composites. The method comprises the following steps in sequence: feeding the raw vegetables into a microwave vacuum tank to undergo microwave treatment; drying, pre-carbonizing, carbonizing, calcining and cooling the pretreated raw vegetables in sequence; controlling combustion to raise the temperature to 700-750 DEG C or 900-1000 DEG C at a rate of 5-15 DEG C/min and maintaining the temperature for 40-80 minutes; then raising the temperature to 1000-1500 DEG C or 1600-1800 DEG C at a rate of 5-15 DEG C/min and maintaining the temperature for 2-8 hours to obtain vegetable carbon; and blending and modifying the base materials such as high impact polystyrene, polyvinyl chloride and ultra-low-density polyethylene with molecular weights of 3-5 million and adding a flame retardant, a toughening agent, a coupling agent, an antioxidant and the obtained vegetable carbon in the processing course. The prepared vegetable carbon plastics have the advantages of excellent toughness, higher rigidity and hardness and good antistatic property and flame retardance.