The invention relates to a method for preparing vinylidene fluoride / N, N-dimethylacrylamide copolymers in a green manner. The vinylidene fluoride / N, N-dimethylacrylamide copolymers are used as separation membrane materials. The method has the advantages that the method is a copolymer synthesis technology based on water / oil two-phase interfacial free radical polymerization, oxidation-reduction reaction is carried out by special Gemini surface active initiators and potassium persulfate at the normal temperature, free radicals are repeatedly generated on interfaces, independent polymerization reaction between vinylidene fluoride and N, N-dimethylacrylamide is initiated, two types of chain segments are connected with each other by initiator molecules in series to form the segmented copolymers, the feed ratios of monomers are extremely consistent with component ratios of the copolymers when pH (potential of hydrogen) values of water-phase media are within the ranges of 4.0-4.5 as proved by experiments, the content of N, N-dimethylacrylamide chain segments in the copolymers is adjustable in the ranges of 5-25 wt.%, polymerization reaction conditions in the modes are mild and controllable, the method is low in energy consumption, other organic solvents or emulsifiers can be omitted, products are pure, and requirements on green chemistry can be completely met.