The invention provides a green preparation method of a vinyl chloride/ dimethyl diallyl ammonium chloride copolymer which is used as a separating membrane material. The green preparation method is a copolymer synthesizing technology based on a water/oil two-phase interfacial free-radical polymerization. The green preparation method is characterized in that a cationic surface active initiator is specifically used and subjected to oxidation-reduction reaction with potassium persulfate under normal temperature; the oxidation-reduction reaction is repeated at a plurality of times so as to generate free radicals on an interface, and the independent polymerization reaction of vinyl chloride and dimethyl diallyl ammonium chloride is initiated; the two chain segments are connected in series through initiator molecules, thus obtaining a segmented copolymer. The experiment shows that when the pH of a water phase medium is 4.5 to 5, the rate of charge of monomer is consistent with the component ratio of the copolymer; the content of dimethyl diallyl ammonium chloride chain segment in the copolymer can be adjusted between 5-16wt.%; the polymerizing reaction under such mode is mild and controllable in condition, low in energy consumption, free of other organic solvents and emulsifiers, and pure in products, and completely meets the requirement of green chemistry.