The invention discloses an active material modified diaphragm and a preparation method thereof. The diaphragm comprises a base diaphragm layer with insulation and supporting functions and an active substance layer capable of reacting with the lithium dendrites. A base diaphragm is a composite diaphragm composed of one or more of a polyethylene film, a polypropylene film, a polyvinylidene fluoridefilm, a polyvinylidene fluoride-hexafluoropropylene film, a polyimide film, a polyamide film, a polytetrafluoroethylene film, a polyacrylate film, a polyvinyl chloride film, a polyethylene oxide film,a glass fiber film, a cellulose film and a non-woven fabric film. The preparation method of the active substance layer comprises the steps that active substances and an adhesive are prepared into mixed slurry, and the mixed slurry is deposited on the base diaphragm. A deposition method comprises the steps of scraping coating, transfer coating, spraying, magnetron sputtering, particle beam sputtering, atomic layer deposition, electron beam evaporation, pulse laser deposition and vapor deposition. The active substance layer is deposited at least on one side of the diaphragm.