The invention relates to a composite diaphragm, consisting of a porous polyimide layer and a montmorillonite layer. The preparation method of the composite diaphragm comprises the following steps: dissolving the polyamide acid in N, N dimethylacetamide, regulating the voltage of the electrospinning device in the range of 15kV to 30kV, and regulating the distance between the receiving disk and thespray-head in the range of 10cm to 30cm; obtaining a uniform film by adjusting the trajectory of the receiving plate or the spray-head; keeping the film at 150 to 200 degrees centigrade for 1 hour inargon atmosphere, and at 400 to 500 degrees centigrade for 3 hour, so as to obtain a film A; weighting the montmorillonite and the polyamide acid by mass ratio of 100:5 to 100:30 and stirring evenly in the N, N dimethylacetamide; selecting the propanol with a weight percentage of 0.1% to 0.5%, adding to the solution and stirring evenly; making the above mixed substance into a solution, coating onthe surface of the film A and performing imidization in the argon atmosphere. The composite diaphragm benefits the diffusion and transmission of electrolyte ions, and is used to improve the safety ofthe battery.