The invention discloses a hydrogen terminal diamond-based two-step method dielectric layer field effect transistor, and a preparation method. A hydrogen terminal diamond epitaxial film 2 is grown on adiamond substrate 1, and a source electrode 3 and a drain electrode 4 are prepared on the hydrogen terminal diamond epitaxial film 2, a dielectric layer 5 is deposited to cover all structures, the dielectric layer 5 is patterned, the source electrode 3, the drain electrode 4 and the dielectric layer 5 therebetween are retained, and a grid electrode 6 is deposited on the dielectric layer 5 betweenthe source electrode 3 and the drain electrode 4; and the dielectric layer is prepared by using a two-step method, the preparation of the dielectric layer is completed in two steps, the dielectric layer is prepared at first by using a low-temperature process to protect a two-dimensional hole gas of the hydrogen terminal diamond from being destroyed, and a high-quality dielectric layer is deposited by using a high-temperature process to improve the device performance, thereby improving the electrical characteristics of the device, an aluminum oxide film prepared by the two-step method can notonly protect the two-dimensional hole gas of the hydrogen terminal diamond maximally, but also can obtain the high-quality dielectric layer aluminum oxide film.