The invention provides a brominated SBS product with a narrow molecular weight distribution and high thermal stability and a preparation method thereof, comprising the following steps: mixing, dissolving, adding dropwise, terminating the reaction and separating, and extracting; said mixing, TBAB and Br 2 The mixture 25.64g and DCM 100g are mixed, obtain material; The TBAB and Br 2 The mol ratio is 1:1, the obtained brominated SBS product of the method of the present invention, 1% thermogravity loss temperature is 230 ℃, 5% thermogravity loss 245 ℃, 10% thermogravity loss 255 ℃, 50% thermogravity loss 280 ℃, bromine The content is 66%. The preparation method of the present invention has stable and easy-to-obtain raw materials, simple process, easy operation, and low cost, which is conducive to popularization and application, and can effectively reduce or eliminate the content of polymers and ultra-high molecular polymers in the copolymer , greatly improving the purity of brominated SBS products, and improving the comprehensive physical and mechanical properties of brominated SBS copolymers.