The invention provides a styrene-butadiene rubber modified asphalt composition and a preparation method thereof. The preparation method comprises the following steps of: grinding styrene-butadiene rubber grains or powder by using special equipment such as a high-shear equipment, colloid mill or the like at the temperature of between 150 and 230 DEG C, and uniformly dispersing in base asphalt to prepare a modified asphalt composition; adding a composite additive in an amount which accounts for 0.01 to 0.2 percent of the total weight of asphalt with the common stirring or stirring by using a high-shear emulsifying machine; and performing cross-linking reaction to prepare a styrene-butadiene rubber modified road asphalt product. The styrene-butadiene rubber modified asphalt has a simple production process; and the asphalt product has higher toughness and tenacity, high high-temperature performance and high low-temperature performance and is suitable for building high-grade highways.