The invention discloses a preparation method of a halogenated monoolefine-conjugated diene copolymer. The method comprises steps as follows: under the cationic polymerization condition, monoolefine shown in a formula II and conjugated diene shown in a formula III are contacted with components in an initiator system in a mixed solvent of alkane and haloalkane, halogenated alkane in a solution obtained through polymerization is replaced with alkane, besides, after unreacted monomers are removed, a product is contacted with a compound containing halogen for a halogenating reaction, the initiator system contains a compound capable of providing carbocation, lewis acid and an activator, and the activator is selected from a compound shown in the formula I-1 and a compound shown in a formula I-2. By means of the method, the polymerization efficiency can be remarkably improved, and the higher polymer yield is obtained; the polymer with higher molecular weight can also be obtained. By means of the method, the polymer redissolving process required with a slurry polymerization method is omitted, and the production process is simplified.