The embodiment of the invention discloses a preparation method for a power device structure, and the method comprises the steps: injecting phosphorus onto a thin film SOI wafer, carrying out high-temperature annealing, activating injected ions, and carrying out heavy doping of an n-type drift region from the surface of the device to the upper surface of a buried oxide layer; carrying out silicon etching till the buried oxide layer, forming a SiO2 dielectric layer window, carrying out SiO2 deposition, carrying out SiO2 etching till the surface of the buried oxide layer, forming a polycrystalline silicon window, and carrying out polycrystalline silicon deposition; carrying out phosphorus injection through a specially-customized photoetching plate, and then carrying out long-time high-temperature annealing; forming a drain electrode, a source electrode, a grid electrode, a gate-oxide, and a polysilicon gate, wherein the polysilicon gate is connected with a gate in the drift region; depositing field oxide SiO2 and metal, carrying out the etching of the metal, and forming source, drain and grid metal, thereby effectively improving the breakdown voltage of a device, and reducing the conduction resistance of the device.