The invention discloses emulsified asphalt for a protective coating, which comprises the following raw materials in parts by weight percent: 30-55 parts of asphalt, 10-20 parts of octadecyl dimethyl benzyl ammonium chloride, 5-10 parts of fatty acid methyl ester ethoxylate, 5-12 parts of sodium carboxymethyl cellulose, 8-15 parts of isobutyl acrylate and 20-60 parts of water. The emulsified asphalt of the invention is used as a protective coating of metal, can form a compact coated film, increases the shielding performance of the emulsified asphalt coating on a metal matrix and further improves the corrosion resisting property of the emulsified asphalt coating; and the invention has simple manufacturing method, easily-obtained raw materials and small environment pollution.