The present invention discloses phonochemical process of preparing benzyl magnesium chloride as Grignard reagent. The process adopts industrial level tetrahydrofuran or ethyl ether as solvent, and performs ultrasonic reaction in initially nitrogen exhausted reactor with ultrasonic source in reaction initiating time of 10-20 sec and yield of 70-82, without need of special treatment on solvent, continuous nitrogen introduction and initiator. The phonochemical process of preparing benzyl magnesium chloride has mild reaction condition, high yield, low cost, easy industrial application and other features.