The invention discloses a preparation method of a depletion mode field effect transistor device, which belongs to the field of semiconductor protection devices, and comprises the following steps: S1, forming a first epitaxial layer on a substrate; S2, growing an oxide layer, and forming at least two column regions in the first epitaxial layer; S3, forming a second epitaxial layer; S4, forming at least two well regions; S5, forming a deep groove, forming a gate oxide layer in the deep groove and on the upper surface of the deep groove, forming a channel between the deep groove and the well region, and forming a polycrystalline silicon layer on the upper surface of the gate oxide layer; S6, forming a first N-type injection region, a first P-type injection region and a second N-type injection region which are connected in sequence; S7, forming a dielectric layer, and forming a corresponding contact hole; S8, depositing source electrode metal and grid electrode metal; and S9, carrying out grinding and thinning treatment, and forming drain electrode metal. The beneficial effects of the technical scheme are that on resistance is low, withstand voltage is high during turn-off, and leakage current is small.