The invention discloses waterborne fireproof coating for a steel structure and a preparation method of the waterborne fireproof coating. The waterborne fireproof coating is mainly prepared from, by weight, 15-23 parts of ammonium polyphosphate, 11-13 parts of melamine, 6-12 parts of pentaerythritol, 7-13 parts of waterborne epoxy resin, 11-15 parts of hydroxyl acrylic acid emulsion, 6-10 parts of phenolic resin, 2-6 parts of montmorillonite, 19-25 parts of deionized water, 1.0-1.8 parts of thickener, 2-4 parts of dispersion agent and 0.14-0.22 part of defoaming agent. The waterborne fireproof coating is long in flame retardance time, high in adhesive strength, less prone to seasoning crack, excellent in fire resistance, nontoxic, safe, environmental friendly, and the like.