The invention relates to biological asphalt and a preparation method thereof. The preparation method comprises steps as follows: 1) inorganic salt is blended in biological oil and mixed uniformly; crude glycerin is added, and the mixture is mixed uniformly; 2) a product obtained in the step 1) is distilled under the normal pressure, and light components and mixed components comprising glycerin andlignin pyrolysis products are separated; 3) water is added to the mixed components, the mixture is mixed uniformly, left to stand and filtered, a water-glycerin mixed phase is removed, and a crude lignin pyrolysis product is obtained; 4) the crude lignin pyrolysis product is heated to the constant weight state, and the biological asphalt is obtained; the water-glycerin mixed phase is distilled under the normal pressure, and separated glycerin is recycled. The lignin pyrolysis product can be effectively separated through pretreatment of biomass pyrolysis oil, and meanwhile, the separation process of conventional distillation is improved, so that the asphalt components in the biomass pyrolysis oil can be sufficiently separated and have excellent usability and ageing resistance.