The present invention relates to an anion-exchange membrane extractor for boric acid separation, which is capable of separating boric acid from the radioactive liquid waste concentrated by evaporation of the radioactive liquid waste produced in the pressurized light water reactor and a method of separating boric acid from the radioactive liquid waste employing the extractor. The onion-exchange membrane extractor for boric acid separation which comprises: an upper cell being equipped with an inlet, an outlet, and fluid-injecting holes; a concentrate cell, an extract cell and a lower cell, each of which is equipped with an inlet, an outlet, fluid-injecting holes, and a supporting shelf on the upper layer; a. water-proofing means for preventing leaking-out of the concentrate and the extract, being inserted into the interface between the said cells; a transporting tube being connected to the inlets and outlets of the said cells for the transfer of the concentrate and the extract between the cells; a supporting screen being held up on the supporting shelf positioned in each of the said cells, holding up the onion-exchange membrane; and, an onion-exchange membrane being supported by the said supporting screen to extract boric acid.