The invention discloses a diamond-magnesium composite heat-radiation material as well as a preparation method and the application thereof. A gradient layer from titanium carbide to a titanium copper alloy is generated on the surface of a diamond to modify and improve interface combination of the diamond and a magnesium substrate, a light diamond-magnesium composite heat-radiation material with high heat conductivity is rapidly synthesized by using a discharge plasma sintering technique, so that the problem of heat radiation caused along with increase of sealing density, of a device of the electronic sealing field, is effectively solved.