The invention relates to an ultrathin water-based epoxy resin expandable fireproof paint and a preparation method thereof, belonging to the technical field of plating metal materials. The fireproof paint comprises water, aqueous epoxy emulsion, a solidification agent, ammonium polyphosphate, a charring foaming agent, a charring accelerant, nano fillers, inorganic fireproof fillers, a dispersing agent, a defoaming agent and a thickening agent. A ball milling machine is adopted for preparing a double-component paint; component A is prepared by two steps of: (1) dispersing the ammonium polyphosphate, the charring foaming agent, the charring accelerant, the nano fillers and the inorganic fireproof fillers in the water by the dispersing agent, with the milling time of 1 to 2 hours; and (2) adding the solidification agent and the defoaming agent for continuously milling for 0.5 to 1 hour; and component B is prepared by one step of: milling the aqueous epoxy emulsion, the thickening agent and the defoaming agent for 0.5 to 1 hour. When the coating thickness of the fireproof paint is smaller than or equal to 1mm, the fireproof time is larger than 90 minutes, the maximum smoke density is smaller than 50, and the adhesion force is larger than or equal to 12MPa. The invention has the advantages of simple preparation process, environmental protection and excellent performance, and can be used for the fire safety protection of the inner parts of large steel structures of buildings, ships, oil storage tanks and the like.