The application discloses a process for preparing watersoluble phosphonooxymethyl derivatives comprising the steps of wherein R-OH represents an alcohol- or phenol-containing drug, n represents an integer of 1 or 2, R 1 is hydrogen, an alkali metal ion, or a pharmaceutically acceptable cation, and R 2 is hydrogen, an alkali metal ion, or a pharmaceutically acceptable cation； wherein the second step comprises reacting the R-O-CH 2 Cl with an excess of phosphoric acid and a base in a suitable solvent.
