The invention relates to a preparation method of dialkyl phosphinic acid. The preparation method comprises the following steps: mixing magnesium metal, halohydrocarbon and a first solvent and carrying out a reaction to obtain a Grignard reagent; adding phosphorus oxychloride into the Grignard reagent and carrying out a reaction to obtain a dialkyl phosphoryl chloride coarse product; and adding an organic acid to hydrolyze dialkyl phosphoryl chloride in the dialkyl phosphoryl chloride coarse product to dialkyl phosphinic acid.