The invention relates to a method for modifying a styrene-butadiene-styrene (SBS) block copolymer. In the method, a sulfydryl-vinyl addition reaction is adopted to carry out modification of various functional groups on SBS, so that the aim of carrying out modification of various functional groups on the SBS is fulfilled. Compared with the prior art, various performances of the obtained modified SBS are improved. For example, compared with the glass-transition temperature of original SBS, the glass-transition temperature of the carboxylic SBS is improved by 46 DEG C. The method disclosed by the invention is simple. The reaction has high efficiency. The method is convenient for large-scale production. The prepared modified SBS has wide application prospect, can be directly used as a polymerwith high performance and can also be used as a modifier of other polymer materials, for example, a compatibilizer of asphalt polymer blend systems such as polycarbonate, polyester, nylon, polyurethane, polyvinyl chloride and the like.