The invention discloses a method for efficiently preparing fatty acid from recycled illegal cooking oil. The method comprises the following steps: S1, taking illegal cooking oil, carrying out primary filtration, adding phosphoric acid, heating, carrying out heat preservation and stirring, then adding water, continuously carrying out heat preservation and stirring, centrifuging and taking an oil phase, and carrying out secondary filtration, so that clear oil liquid is obtained; S2, adding water into the clear oil liquid obtained in the S1, uniformly mixing, then adding a catalyst and an adsorbent, heating, carrying out heat preservation and stirring, introducing hydrogen, continuously carrying out heat preservation and stirring, carrying out reduced pressure evaporation, and collecting evaporated gas, so that the fatty acid is obtained. The method disclosed by the invention has the advantages that an appropriate catalyst and adsorbent are selected, and an appropriate preparation technology is combined, so that yield of the fatty acid is greatly increased, preparation period is shortened, and preparation efficiency is improved.