The invention relates to a halogenated polymer. The polymer has a structure of S1-I/B-S2 before halogenation, wherein S1 and S2 are polystyrene block, I/B represents a random compolymerization segment. In the halogenated block copolymer, 0.4-80% of carbon-carbon double bonds in an isoprene monomer unit are halogenated, and the ratio of the halogenation degree of carbon-carbon double bonds in a butadiene monomer unit to the halogenation degree of carbon-carbon double bonds in the isoprene monomer unit does not exceed 0.01. The invention also relates to a method for preparing the halogenated multiblock copolymer. The halogenated multiblock copolymer is capable of effectively improving the compatibility, also rising of the glass transition temperature is not too obvious, and thus the bonding performance is influenced when the halogenated multiblock copolymer is applied to a pressure-sensitive adhesive, and especially the peeling strength of the pressure-sensitive adhesive is influenced.