The invention discloses modified terpene resin and a preparation method thereof, and relates to the technical field of preparation of forestry chemical products. Directed at defects of terpene resin and existing modified terpene resin in the market at present, the modified terpene resin is a polymer obtained by copolymerization of three monomeric substances including 20wt%-80wt% of pinene, 10wt%-50wt% of cis-1, 3-pentadiene and 5wt%-40wt% of alpha-methylstyrene under the action of a catalyst and an antioxidant. The modified terpene resin is characterized by low color and luster, high softening point, low wax cloud point, good compatibility, excellent anti-aging performance and the like. When applied to the adhesives, the modified terpene resin can substantially improve the use performance of adhesives, and not only increases the initial adhesion, but also apparently improves the persistent adhesion and peeling strength. The preparation method disclosed by the invention is easy, the reaction condition is easy to control, the production cost is low, the polymerization yield is high, and the industrial production is facilitated.