The invention provides a preparation method for a graphene phenolic resin-based composite fibrous membrane applied to an electrode of a capacitor. The method comprises the following steps: with biomass active carbon fibers as a skeleton and emulsion beads as a template, adding the skeleton, the template and graphene oxide hydrosol into a mixed solvent composed of water and an organic solvent so asto form a suspension; subjecting the suspension to deposition and compounding so as to obtain a composite flexible membrane; laying the composite flexible membrane into a graphite fixture and carrying out hot pressing treatment in a vacuumized state or inert atmosphere so as to obtain a 3D macroporous bubble-like composite membrane; laying the 3D macroporous bubble-like composite membrane in prepreg phenolic resin; and successively carrying out vacuum-pumping at room temperature, hot pressing, and dipping and activation in alkali liquor so as to obtain the graphene phenolic resin-based composite fibrous membrane. The graphene phenolic resin-based composite fibrous membrane provided by the invention allows the electrode of the capacitor to have the advantages of high energy density and good cycling stability.