The invention relates to a preparation method for dexamethasone sodium phosphate. The preparation method comprises the following steps: a ring-opening reaction is carried out, namely, dexamethasone acetate epoxide is employed as an initial raw material, HF and DMF are added, a reaction is performed for 3h, a ring-opening reaction is carried out and a dexamethasone acetate solution is prepared; recrystallization is carried out, namely, methanol is added in the dexamethasone acetate solution, recrystallization is carried out, and dexamethasone acetate is prepared; base catalysis hydrolysis is carried out, namely, dexamethasone acetate is added in Na2CO3 and methanol, a reaction is carried out for 10min, dexamethasone is prepared; pyrophosphoryl chlorine esterification is carried out, namely, dexamethasone prepared in the third step is reacted with pyrophosphoryl chlorine and THF, and dexamethasone phosphate ester is prepared; a neutralization salt forming reaction is carried out, namely, the dexamethasone phosphate ester obtainedin the fourth step is reacted with NaOH and methanol at a reaction temperature of 20 DEG C-30 DEG C for 1h, and dexamethasone sodium phosphate is prepared. The steps are simple, raw materials are easily available, the reaction conditions are mild, the method is suitable for industrial production, and the cost is low.