The invention provides a dissolvent thermal synthesis Method for preparation of maleic anhydride grafting ethene-octene copolymer, adding ethane-octene resin, maleic anhydride, initiator and monomer and dissolving them in solvent, controlling reaction temperature above the solvent's boiling point, carrying through reaction in the supercritical state of systerm, making ethene-octene contacts with reactant sufficiently, then washing and desiccation, getting the grafted copolymer of ethene-octene maleic anhydride. The getted maleic anhydride grafting ethene-octene copolymer is used as compatible reagent of ethene-octene and polycarbonate, polyester, nylon, polyurethane, polyvinyl chloride blending, viz. getting composite material of better combination property. This invention's method is simple, the grafting rate is high, the mass production is easy and the application foreground is wide.