A graft copolymer, composed of maleic anhydride as side group and chlorinated polyvinyl chloride as backbone, was prepared via in situ chlorinating graft copolymerization. The effect of graft degree of the graft copolymer on the chain structure is discussed through proton nuclear magnetic resonance. The results of differential scanning calorimetry and thermogravimetic analysis showed glass transition temperature of the graft copolymer did not change with increase in graft degree, but thermal stability of the product improved much with increase in graft degree. The results of gel content measurements showed the increasing graft degree was favorable to prevent cross-linking reaction of the product.