The invention relates to a gallium nitride device based on a diamond substrate and a preparation method of the gallium nitride device. The preparation method comprises the following steps: growing a diamond layer on the first surface of a gallium nitride material layer; growing an AlN nucleating layer on the second surface of the gallium nitride material layer; growing a GaN buffer layer on the AlN nucleating layer; and growing an AlGaN barrier layer on the GaN buffer layer. According to the preparation method, the diamond layer is directly grown on the first surface of the gallium nitride material layer, the introduction of the dielectric layer is avoided, the influence of thermal resistance of the dielectric layer material is eliminated, and the diamond is in direct contact with the gallium nitride material, so that the gallium nitride device directly utilizes the diamond with higher thermal conductivity to dissipate heat, and the heat dissipation problem of a gallium nitride high-power device is effectively solved.