The invention relates to a hard-asphalt extender and a preparation method thereof. the method comprises the following steps: 1, smashing biological substances; 2, performing pyrolysis on the obtained biological substances to obtain biological oil; 3, mixing the obtained biological oil and an inorganic-salt solution, performing stirring in cold water bath, and performing vacuum filtration to obtain a non-water-soluble component; and 4, performing heat treatment on the obtained non-water-soluble component in a nitrogen atmosphere to obtain the hard-asphalt extender. According to the invention, the ordinary biological substances such as waste wood residues and waste rice hulls are converted into the biological oil, a physical separation method used for asphalt material components in the biological oil is improved, and therefore usage performance of the biological asphalt extender is improved and an optimal usage range of biological asphalt which is used as the extender is decided.