A composite dielectric structure of a gallium nitride electronic device comprises a low-interface-state dielectric insertion layer and a high-breakdown electric field dielectric layer, the low-interface-state dielectric insertion layer grows on the surface of the gallium nitride electronic device, and the high-breakdown electric field dielectric layer grows on the low-interface-state dielectric insertion layer. The invention further provides a preparation method of the composite dielectric structure. The method comprises the following steps: placing a gallium nitride electronic device in a machine table, adjusting the temperature of the machine table to a first preset temperature, adjusting the power to first preset power; cleaning the surface of the gallium nitride electronic device by using plasma; and adjusting the temperature of the machine to a second preset temperature and adjusting the power to a second preset power, growing a low-interface-state dielectric insertion layer on the surface of the gallium nitride electronic device, adjusting the temperature of the machine to a third preset temperature and adjusting the power to a third preset power, and growing a high-breakdownelectric field dielectric layer on the low-interface-state dielectric insertion layer. The problem of surface interface defects of the gallium nitride electronic device can be effectively solved.