The invention provides a phosphate binding agent modified by 1-hydroxyethylidene-1,1-diphosphonic acid and a preparing method of the phosphate binding agent. The phosphate binding agent modified by 1-hydroxyethylidene-1,1-diphosphonic acid is mainly prepared from, by mass percentage, 51.59-52.45% of industrial phosphoric acid, 11.83-12.03% of aluminum hydroxide powder, 0.61-0.62% of silicic acid,1.83-1.86% of boric acid, 1.22-1.24% of light magnesium oxide, 0.76-2.4% of 1-hydroxyethylidene-1,1-diphosphonic acid and 30.53-31.04% of water. According to the phosphate binding agent modified by 1-hydroxyethylidene-1,1-diphosphonic acid, 1-hydroxyethylidene-1,1-diphosphonic acid is adopted as a modifier, and even under the condition of a small addition amount, the viscosity of the binding agentcan be significantly lowered, the strength of a prepared sand sample can be significantly improved, added raw materials participate in the reaction to the greatest extent, few residues are generated,water loss is reduced to guarantee that the binding agent has low viscosity, and the fluidity of foundry sand is improved.