The invention discloses a method for preparing sulfonate by recycled styrene tar. According to the method, a sulfonating agent is added into the styrene tar, and the styrene tar is converted into styrene tar sulfonate through sulfonation reaction. According to the method, the sulfonation degree of the styrene tar is controlled by controlling sulfonation reaction conditions such as molar ratio, reaction time and reaction temperature of the sulfonating agent and the styrene tar, the sulfonate yield of the styrene tar is improved, and the styrene tar sulfonate have sulfonic acid groups and hydrophilcity and can serve as surface active agents such as cement water-reducing agents.