The invention discloses a synthesis method for herbicide oxaziclomefone intermediate 3,5-dichlorobenzyl chloride, and belongs to the field of pesticide preparation. The synthesis method is characterized in that 3,5-dichlorobenzoic acid, a composite hydrogen compound and metal chloride react under action of polyethylene glycol to obtain 3,5-dichlorobenzyl chloride. A reaction process comprises thefollowing steps: 1) adding 3,5-dichlorobenzoic acid, a solvent A and the composite hydrogen compound into a reaction kettle under a protective gas condition, heating to 40-60 DEG C to react for 5-12 minutes, adding metal chloride, polyethylene glycol and alkaline water, starting ultrasonic assistance, setting power and frequency, controlling the temperature to be 90-110 DEG C, keeping reaction pressure at 1.5-2 barometric pressure, and ending reaction after 2-4 hours; and 2) after the system is cooled, adding a solvent B which is about 1-2 times the volume of reaction liquid, filtering to remove insoluble substances, layering, drying an organic phase with a drying agent, concentrating and re-crystallizing to obtain 3,5-dichlorobenzyl chloride. The synthesis method is fewer in step, less inside reaction, high in yield, less in pollution discharge and safe to operate.