The invention discloses hard asphalt 20# and a preparation method thereof. The preparation method comprises the following steps: (1) raw oil is subjected to deep vacuum distillation, and vacuum residual oil in the raw oil is collected, wherein the deep vacuum distillation process is that the raw oil is distilled through an atmospheric tower to obtain atmospheric residual oil and the atmospheric residual oil is distilled through a vacuum distillation tower; (2) the vacuum residual oil is oxidized in an oxidation tower, and the hard asphalt 20# is obtained. According to the preparation method, the temperature of an outlet of a vacuum furnace in the deep vacuum distillation process is increased, so that a softening point of the vacuum residual oil prepared by the vacuum distillation tower is increased, the needle penetration degree is reduced, and the needle penetration can be conveniently oxidized to obtain the hard asphalt with better oxidation resistance; the hard asphalt 20# can be used for partially replacing asphalt 70# or SBS (styrene butadiene styrene) modified asphalt to be laid on an asphalt pavement, has high-temperature stability and anti-rut capacity, prolongs the service life of the pavement, does not have an obvious low-temperature cracking phenomenon, and is good in low-temperature performance; the hard asphalt 20# has social and economic benefits when being used for pavement construction.