The invention provides a modified dielectric fluorine-containing polymer, and a film material and a preparation method thereof, wherein the preparation method of the modified dielectric fluorine-containing polymer comprises the following steps: step 1, adding a surfactant and a dielectric fluorine-containing polymer to an alkaline solution to obtain a mixed solution; and step 2, enabling the dielectric fluorine-containing polymer in the mixed solution to be subjected to partial elimination reaction to obtain the modified dielectric fluorine-containing polymer, wherein the modified dielectric fluorine-containing polymer contains carbon-carbon double bond. The dielectric fluorine-containing polymer and the film material described according to the embodiment of the invention are simple in preparation process, low in cost, suitable for industrialized production, and the modified dielectric fluorine-containing polymer prepared according to the method has good dielectric and breakdown resistant performances.