The invention relates to the field of high polymer materials and particularly relates to a preparation method of an organic siloxane modified chlorinated polypropylene adhesive. According to the preparation method, chlorinated polypropylene is modified by virtue of an organic siloxane modifying agent and acrylate monomers. The preparation method comprises the following specific steps: (a) preparing a grafting solution from the organic siloxane modifying agent and the acrylate monomers; (b) preparing a chlorinated polypropylene solution; and (c) reacting by virtue of the grafting solution and the chlorinated polypropylene solution, and separating and purifying reactants, so as to obtain the organic siloxane modified chlorinated polypropylene adhesive. The preparation method has the beneficial effects that the process is simple, the reaction is easily controlled, the cost is low, the grafting rate of chlorinated polypropylene is high, and both the adhering force and waterproof effect of the prepared organic siloxane modified chlorinated polypropylene adhesive are enhanced.