The hydroxyl branched-functionalized styrene-butadiene-styrene triblock copolymer (SBS-g-OH) was prepared using thiol-ene radical addition reaction with commercial SBS(raw material), 2-mercaptoethanol(MCH, modifying agent), azodiisobutyronitrile(AIBN, initiator), 1,4-dioxane(DOA, solvent). The reliability of this reaction was proved by the characterization of fourier transform-infrared spectroscopy (FT-IR), 1H-nuclear magnetic resonance(1H NMR) spectroscopy, and thermogravimetric analyzer(TGA). The effects of reaction time, temperature, MCH amount on grafting functionality degree were investigated, furthermore, the effects of grafting functionality degree on the C=C units of PB chains and molecule weight of SBS-g-OH were also investigated. Hydroxyl was mainly grafted onto the 1,2-C=C units of PB chains in SBS at the scale of functionality degree below 33.27 mol%, based on a certain amount of materials(SBS: MCH: AIBN: DOA = 10: 2: 0.02: 200 (g)), functionality degree was increased with increasing reaction time and temperature at the temperature ranged from 60 to 80 oC and time ranged from 1 to 15 h. A little number of hydroxyl was grafted onto the 1,4-C=C units of PB chains at the functionality degree of 38.57 mol%, and the 1,4-C=C units began to scissor, which made the molecular weight of SBS reduced, at the functionality degree of 41.27 mol%. The phenomenon of gel occurred at the SBS(g)/MCH(g) ranged from 0.05/1 to 0.2/1. Finally, the dispersivity of SBS-g-OH with different functionality degree in modified asphalt, and the storage stability of modified asphalt were investigated. The dispersivity was improved with increasing functionality degree at the scale of functionality degree below 33.27 mol%, especially, at the functionality degree of 24.07 mol%, segregation degree and storage stability of modified asphalt was most improved by SBS-g-OH.