NOVELTY - Preparing sodium ibandronate monohydrate comprises (i) reacting 3-(N-methylpentylamino)propionic acid hydrochloride, phosphorus trichloride and phosphorous acid in chlorobenzene, removing supernatant, adding hydrochloric acid solution, reacting, filtering, concentrating, dissolving concentrate in water, adding extraction agent and extracting, allowing to stand for stratification, removing organic layer and concentrating the lower aqueous layer to obtain ibandronic acid; and (ii) dissolving ibandronic acid in water, adjusting pH using 30% sodium hydroxide solution, crystallizing, filtering and drying.    USE - The method is useful for preparing sodium ibandronate monohydrate (claimed).    ADVANTAGE - The method improves the yield by reducing large number of purification steps, shortens the production time and reduces human and environmental hazards. The product meets the impurity control standards and contains less amount of chloride and phosphite.    DETAILED DESCRIPTION - Preparing sodium ibandronate monohydrate comprises (i) reacting 3-(N-methylpentylamino)propionic acid hydrochloride, phosphorus trichloride and phosphorous acid with a molar ratio of 1:2.5:2.5 in chlorobenzene, where the concentration of 3-(N-methylpentylamino)propionic acid hydrochloride in chlorobenzene is 0.1-0.3 g/l, removing supernatant, adding 0.1 mol/l hydrochloric acid solution of 2-4 times chlorobenzene volume, stirring and reacting, filtering, concentrating the filtrate under reduced pressure, dissolving the concentrate in water, adding extraction agent and extracting, allowing to stand for stratification, removing the organic layer, and concentrating the lower aqueous layer to obtain ibandronic acid, where the volume ratio of water and hydrochloric acid solution is 1:3-1:5, and volume ratio of extraction agent and water is 4:1; and (ii) dissolving ibandronic acid in water, adjusting pH to 4-4.8 using 30% sodium hydroxide solution, adding crystallization solvent, crystallizing completely, filtering and drying, where the mass ratio of water and 3-(N-methylpentylamino)propionic acid hydrochloride is 3:1-5:1, and the volume ratio of crystallization solvent and water is 1:1-3:1.