The invention provides modified high-viscosity asphalt and a preparation method thereof. The modified high-viscosity asphalt is prepared from, by mass, 88.6%-91.5% of pavement petroleum asphalt, 6.5%-8.5% of butadiene-styrene block copolymer, 1.0%-1.4% of low molecular weight polymer synthetic resin with the molecular weight of 1,480-2,200, 0.8%-1.2% of high molecular weight polymer synthetic resin with the molecular weight of 4,800-8,000 and 0.2%-0.3% of stabilizer. The modified high-viscosity asphalt has the advantage of being good in storage stability, the polymer resin which is slow in separation and is not degradable is used for compound modification, the toughness and visco-toughness of the modified pavement petroleum asphalt are good, the 60-DEG C dynamic viscosity is especially prominent, exceeds 110,000 Pa.S, and is far larger than that, being 60,000-80,000 Pa.S, of commercially available high-viscosity modified asphalt, and the anticorrosion performance is also good.