The invention discloses modified asphalt and a preparation method thereof. The method comprises the following steps: S1, mixing high-temperature asphalt and heavy oil to form a first mixture, wherein the softening point of the high-temperature asphalt is 115-160 DEG C, the beta resin content is 5-15 wt%, and the ash content is no more than 0.5 wt%; S2, mixing the first mixture, crosslinking agent and a catalyst to form a second mixture; and S3, performing crosslinking polymerization reaction on the second mixture at 80-120 DEG C to obtain the modified asphalt. According to the invention, the high-temperature asphalt used as a raw material and the heavy oil are thoroughly mixed and molten, are thoroughly mixed with the crosslinking agent and the catalyst, and are subjected to crosslinking polymerization reaction at a low temperature, thus preparing the modified asphalt which has low softening point and increased beta resin content and can meet the requirements of the market for medium-and-high-temperature modified asphalt; and the modified asphalt has favorable binding property and can be used as medium-and-high-temperature binding agent asphalt and impregnated asphalt required in the market.