Brominated block copolymer serving as flame retardant and preparation method of brominated block copolymer

The invention relates to a brominated block copolymer. The brominated block copolymer is a basic block copolymer shown in a formula S -B -I-B -S before being brominated, wherein S and S independently represent styrene blocks respectively, B and B independently represent butadiene blocks respectively, and I represents an isoprene block; after the basic block copolymer is brominated, at least 40% by mole and at most 95% by mole of unsaturated carbon-carbon double bonds contained in the isoprene block I are brominated, and at least 5% by mole and at most 40% by mole of unsaturated carbon-carbon double bonds contained in the butadiene blocks B and B are brominated. The invention further relates to a method for preparing the brominated block copolymer. The method comprises the step that a solution of the basic block copolymer shown in the formula S -B -I-B -S reacts with a brominating agent. The brominated block copolymer has the excellent heat stability, the good compatibility with other copolymers and the good processing property. According to the method, preferential selective brominating on isoprene monomer units contained in the basic block copolymer shown in the formula S -B -I-B -S can be achieved.