The invention belongs to the field of inorganic chemical technology, and particularly relates to a method for extracting boric acid from boron-containing magnesium chloride solution by using an acidification-extraction method. The method is characterized in that: (a) the process flows comprise acidification, filtering, and multiple extraction, and also have four processes, namely thermosol colorization, slag removal by filtration, cold crystallization filtration, and evaporation, and then a boric acid product is obtained after the crystallization filtration; and (b) the process conditions comprises that hydrochloric acid or sulfuric acid is added into boron-containing magnesium chloride brine in acidification, thereby leading the pH-value of brine to be 0-3; the acidification temperature is room temperature; the acidification time is 10-60 minutes; the mass percent of crude boric acid: industrial hydrogen peroxide: active carbon or fly ash in thermosol colorization is 100:0.01-5:0.5-5; and the heating temperature in thermosol colorization is 80-95 DEG C. The method is fully applicable to the industrial production need of the existing salt lake brine boron extract, greatly lowers the production cost and equipment investment, and lays down solid technology basis for the large-scale industrial salt lake brine extract.