A method for preparing a power diode, including: providing a substrate (10), growing a N type layer (20) on the front surface of the substrate (10); forming a terminal protecting ring; forming an oxide layer (30), knot-pushing to the terminal protecting ring; forming a gate oxide layer (60), depositing a poly-silicon layer (70) on the gate oxide layer (60); depositing a SiO2 layer (80) on the surface of the poly-silicon layer (70) and a oxide layer (50); forming a N type heavy doped region (92); forming a P+ region; removing a photoresist, implanting P type ions using the SiO2 layer (80) as a mask layer, and forming a P type body region; heat annealing; forming a side wall structure in the opening of the poly-silicon layer (70), the gate oxide layer (60) being etched, and removing the SiO2 layer (80); and processing a front surface metallization and a back surface metallization treatment. According to the method for preparing the power diode, by adjusting the isotropy etching level of the SiO2 layer and the ion implanting dose and energy, the threshold voltage of a DMOS structure can be adjusted, and the adjustment of the forward voltage drop for the device can be achieved.