Riboflavin (VB2) 5-phosphate monosodium salt plays an important role in various enzyme reactions in the living organism.It is useful for medicines, food additives and animal feeds.Riboflavin cyclic phosphoridate (Ⅱ) was prepared by reacting riboflavin with phosphoryl chloride in the presence of water and pyridine in acetonitrile.After separating and decomposing the intermediate of riboflavin cyclic phosphoridate (Ⅱ), subsequently alkalization of riboflavin 5-phosphoric acid (Ⅲ), the riboflavin 5-phosphate monosodium salt was obtained.A process has been developed to produce 5-phosphate monosodium salt (Ⅳ) in high yield and low cost.