NOVELTY - Production of ethane-1-hydroxy-1,1-diphosphonic acid involves forming reaction mixture by injecting tetraphosphorus hexoxide to acetic acid in preset ratio, while controlling the reaction mixture at 60-200 degrees C to obtain ethane-1-hydroxy-1,1-diphosphonic acid condensate, hydrolyzing ethane-1-hydroxy-1,1-diphosphonic acid condensate of reaction mixture by adding water, and maintaining the reaction mixture at 100-200 degrees C to obtain aqueous solution comprising ethane-1-hydroxy-1,1-diphosphonic acid.    USE - Production of ethane-1-hydroxy-1,1-diphosphonic acid used for preparing sodium salt, potassium salt and ammonium salt, and used as scale inhibitor, dispersing agent, sequestering agent and corrosion inhibitor (all claimed) used in circulating cool water system, oil field, and low-pressure boiler used in electric power, chemical industry, industrial water treatment, metallurgy, and fertilizer.    ADVANTAGE - The method provides ethane-1-hydroxy-1,1-diphosphonic acid having high selectivity by safe, economical and environmentally-friendly process.    DETAILED DESCRIPTION - Production of ethane-1-hydroxy-1,1-diphosphonic acid or its salt involves forming reaction mixture by gradually injecting tetraphosphorus hexoxide under optimal mixing conditions below the surface of liquid phase comprising acetic acid, while controlling the reaction mixture at 60-200 degrees C to obtain ethane-1-hydroxy-1,1-diphosphonic acid condensate, hydrolyzing ethane-1-hydroxy-1,1-diphosphonic acid condensate of reaction mixture by adding water, and maintaining the reaction mixture at 100-200 degrees C to obtain aqueous solution comprising ethane-1-hydroxy-1,1-diphosphonic acid. The molar ratio of acetic acid to tetraphosphorus hexoxide is 4-15. The molar ratio of water to tetraphosphorus hexoxide is 2 or more. The reaction is carried out under controlled thermal conditions, by avoiding the formation of phosphine and low oxides of phosphorus and accumulation of phosphite intermediate.