Preparation method of adefovir

The invention discloses a preparation method of adefovir. The preparation method comprises the following steps: adding 9-[2-(diethoxylphosphorylmethoxyl)ethyl]adenine into a water solution of HBr, heating to carry out reactions, cooling, and adjusting the pH to 2-3.5 to precipitate white solids namely white adefovir. The preparation method has the advantages that during the hydrolysis of 9-[2-(diethoxylphosphorylmethoxyl)ethyl]adenine, the step of evaporating organic solvents is eliminated, the reaction time is reduced, thus the preparation time is shortened; trimethyl iodo-silane, trimethyl bromo-silane, or trimethyl chloro-silane is replaced by a water solution of HBr, and the cost is reduced by nearly 100 to 400 yuan. Secondary solvent acetonitrile is not used, influence of pollution and solvent residue on drug quality is reduced; trimethyl bromo-silane is easy to decompose and is harmful to human body, and after HBr substitution, the harm to workers is largely reduced.