The invention discloses a high-viscosity asphalt modifying agent, a preparation method and a preparation method of modified asphalt .The high-viscosity asphalt modifying agent is prepared from, by mass, 40-70 parts of a thermoplastic styrene butadiene rubber polymer, 15-45 parts of modified styrene butadiene rubber, 5-20 parts of regenerated polyethylene, 10-45 parts of a regenerated ethylene-vinyl acetate copolymer, 0-5 parts of a cross-linking agent and 0-5 parts of a stabilizing agent .The high-viscosity asphalt modifying agent obtained through compositing according to the specific proportion and processing technology can improve the high-low temperature performance and aging resistance of matrix asphalt, is simple in preparation technology and good in economic performance and can prolong the service life of a drainage asphalt pavement by being further applied to paving of the drainage asphalt pavement.