Abstract: The present invention relates to a method for halogenating an isobutene-isoprene copolymer by using an organic hypochlorite. By using the method of the present invention, a hydrogen halide byproduct may be rapidly recycled into a halogenating agent and reused. As a result, a halogenated isobutene-isoprene copolymer having the high content of secondary allyl halogen functional groups may be prepared by increasing the participation ratio of the halogenating agent in reaction and suppressing the rearrangement of the secondary allyl halogen functional groups. The halogenated isobutene-isoprene copolymer prepared according to the present invention may be effectively used in the crosslinking reaction with other rubber, and is industrially useful.