The invention discloses modified asphalt and a preparation method thereof. The method comprises the following steps: S1, mixing high-temperature asphalt, heavy oil, crosslinking agent and a catalyst to form a mixture, wherein the softening point of the high-temperature asphalt is 115-160 DEG C, the beta resin content value is 5-15 wt%, and the ash content value is no more than 0.5 wt%; S2, heating to melt the mixture, thus obtaining a molten mixture; and S3, performing crosslinking polymerization on the molten mixture at 80-120 DEG C to obtain the modified asphalt. According to the invention, the heavy oil and crosslinking agent are added into the high-temperature asphalt used as a raw material, slow heating is performed to obtain molten asphalt, and then, crosslinking polymerization reaction is performed on the high-temperature asphalt under the action of the catalyst, thus obtaining the modified asphalt which has low softening point and high beta resin content; and the modified asphalt having high beta resin content has favorable binding property and can be used as medium-and-high-temperature binding agent asphalt and impregnated asphalt required in the market.