Method for preparing methyl boric acid

The invention discloses a method for preparing methyl boric acid. The method includes the steps that silica-base-protected methyl bromide and magnesium react in 2-methyl tetrahydrofuran to generate a Grignard reagent, boric acid ester is added in at low temperature, and after the reaction is finished, silicane-protected methyl boric acid is obtained through hydrochloric acid hydrolysis; an intermediate continues to react with tetra-n-butyl ammonium fluoride to remove silica bases, and then methyl boric acid tripolymer is distilled out. After being added into water, the tripolymer can be hydrolyzed into the methyl boric acid through stirring at the room temperature. The synthetic method is simple and reliable, it is avoided that potential safety hazards are amplified, the separation problem of the water-soluble methyl boric acid is solved, the purity of the obtained methyl boric acid can reach over 98%, and a new way is provided for producing the methyl boric acid in an amplified mode.