The present invention discloses a composite positive osmosis membrane and a preparation method thereof, wherein the positive osmosis membrane is formed by compounding a polymer and graphene compositelayer and a highly porous substrate lower layer, the polymer is a copolymer formed by carrying out free radical polymerization on an isopropyl acrylamide monomer and a N,N-methylene diacrylamide monomer, the highly porous substrate is a porous aromatic polymer film, a mass ratio of the two monomers is 1:9-9:1, and the mass of the graphene is 2.5-70% of the mass of the copolymer. The preparation method comprises precursor solution preparation, film coating, heat treatment, and other steps. The positive osmosis membrane of the present invention has high water flux and high salt rejection rate.