The invention relates to a preparation method of a flexible supercapacitor, and a preparation method of an electrode and a diaphragm thereof. The method comprises the following steps: designing the sizes of the electrode and the diaphragm, and customizing a forming mould, respectively loading a gel state electrode mixture and a gel state diaphragm mixture into the forming mould, pushing and extruding a gel state material from one side of the forming mould, after the gel state material is extruded from the other side to a fixed thickness, slicing the gel state material with the extruded fixed thickness by using a cut-off knife to obtain a flexible electrode and a flexible diaphragm, then stacking the flexible electrode and the flexible diaphragm to form a capacitor cell, and drying and packaging the capacitor cell to obtain the flexible supercapacitor. By adoption of the above method provided by the invention, the trimming loss of the flexible electrode and the flexible diaphragm in the cutting process is avoided; the flexible electrode has good tensile strength, high deformation rate and high electrical conductivity; the flexible diaphragm has good elasticity, large tensile strength and high deformation rate; and the flexible supercapacitor product has high production efficiency, good bending performance and wide electrochemical window.