The invention belongs to the technical field of preparation of chemicals, and particularly relates to a device and method for preparing hydrogen peroxide by using plasma discharge. The device is composed of a pulse power source, a water storage tank, a detachable outer wall structure, an ultrasonic atomizer, an array-type discharge electrode, a gas outlet diversion fan and a water mist condensation structure. The ultrasonic atomizer is utilized to generate water mist. The gas inlet hole, gas outlet diversion fan and integrally closed structure of the device are utilized to form an upward gas flow, and the introduced oxygen carries the misty water drops to pass through the discharge region upwards. Under the action of the plasma, double-dielectric barrier discharge occurs to generate the hydrogen peroxide. By using water as the raw material, the preparation process is simple and safe, and no pollutant is generated in the hydrogen peroxide production process, thereby implementing the clean mass chemical production technique of the green oxidizer in deed. The device and method are also suitable for flexible preparation of hydrogen peroxide in the fields of daily necessities, industry and medicine.