The invention relates to a method for synthesizing a phosphorus-free corrosion-inhibition scale inhibitor for water treatment from natural products. The method uses maleic anhydride and other unsaturated carboxylic acids to carry out graft modification on natural products, including papermaking waste residue product lignosulphonate, starch and derivatives thereof, other polysaccharides and derivatives thereof, or humic acid, tannic acid and the like. The phosphorus-free corrosion-inhibition scale inhibitor comprises the following raw materials in portions by weight: 100-500 portions of unsaturated carboxylic acid, 20-400 portions of natural product, 0-100 portions of concentrated sulfuric acid, 0.001-50 portions of catalyst, 10-400 portions of oxydol and 100-2,000 portions of water. The product of the invention does not contain phosphorus or nitrogen, has favorable effects of corrosion and scale inhibition in a circulating cooling water system, and is an environmental-protection corrosion-inhibition scale inhibitor.