The invention discloses a modified diaphragm. The modified diaphragm comprises a diaphragm substrate, and an electronic conductive coating layer and an inorganic hard coating layer positioned on the two surfaces of the diaphragm substrate respectively, wherein the electronic conductive coating layer is a mixed coating layer with graphene and a porous carbon material; and the inorganic hard coating layer is a nitrogen-carbon material coating layer, a nitrogen-silicon material coating layer, a carbon-boron material coating layer or a silicon-carbon material coating layer. The preparation method comprises the steps of uniformly coating the two surfaces of the diaphragm substrate with the electronic conductive coating layer and the inorganic hard coating layer respectively, and drying the two surfaces respectively obtain the modified diaphragm. The modified diaphragm has high diffusion and absorption capacity for an organic electrolyte, high ionic conductivity, and an obvious adsorption and blocking effect on poly-sulfur-lithium; in addition, the modified diaphragm can further restrain a shuttle effect, prevent battery short-circuit, effectively restrain the growth of negative electrode metal lithium dendrites in a cyclic process, prevent from punching the diaphragm, and protect the lithium negative electrode, so that the problem that the diaphragm is punched after dendrites formation is solved, and the safety performance of the battery is improved.