The invention relates to an improvement of treatment technology for byproduct of high boiling residues produced in the production process of dimethyl phosphate, in particular to a method of utilizing the high boiling residues to produce orthophosphorous acid and byproduct of chloromethane. The method utilizes the following steps 1) mixing the high boiling byproducts of dimethyl phosphate with dense hydrochloric acid, heating for reaction, 2) adding phosphorus trichloride into solution after heating reaction, when the heating reaction is over, lowering the temperature and crystallizing, filtering to get orthophosphorous acid crystal. The invention adds phosphorus trichloride, consumes the redundant moisture in the system, simultaneously produces hydrogen chloride, thereby leading the reaction temperature not too high, and shortening the reaction time, and simultaneously omitting the operation of vacuum decompression concentration unit, lowering the energy consumption, and recycling high boiling residues.