The invention discloses a surface water-based preserving agent for GCr15 steel. The surface water-based preserving agent is composed of, by weight, 20-24% of deionized water, 14-18% of corrosion inhibitors, 11-16% of 10% tannic acid solutions, 2-3% of potassium sorbate, 2-4% of sodium carbonate, 6-9% of triethanolamine, 2-3% of polyisobutene, 3-6% of guar gum, 2-5% of sesbania gum, 10-15% of mixed alcohols, 2-6% of strong phosphoric acid, 3-6% of grape saccharin, 2-3% of wetting agents and 1-2% of defoaming agents. According to the surface water-based preserving agent, the selected materials are superior, the scientific processing method is adopted, and rust protection and film formation act together; the deionized water serving as a dispersion medium is matched with the other raw materials, the environment friendliness is high, and pollution is avoided. In addition, the surface water-based preserving agent is free of methylbenzene, ethylbenzene, dimethylbenzene, chromic acid, sodium nitrite and formaldehyde, low in volatile organic compound (VOC), free of pungent smells and harmless to the human body, can be easily absorbed by deep structures, can effectively protect the surface of the steel against corrosion, cavitation and scale formation, can form a protective film layer, can delay the time of generating rusts on the surface of the steel and has remarkable environmental benefits, economic benefits and social benefits.