The invention provides a preparation method of maleic anhydride modified epoxy phosphate EMP with high purification precision and convenience in operation. The preparation method comprises the following steps: reacting n-butyl alcohol and MA (maleic anhydride) to prepare a maleic anhydride monoester: putting measured n-butyl alcohol, MA and tetrabutylammonium bromide into a four-neck bottle with a stirrer and a temperature gauge, slowly heating to 60-80 DEG C, and performing heat-preservation reaction for 3 hours in the temperature range; 2, preparing maleic anhydride modified epoxy resin: adding measured E-51 epoxy resin into the monoester, heating to 110-130 DEG C, performing heat-preservation reaction for about 3 hours, sampling and measuring an acid value, stopping reaction until the acid value is less than 5mg KOH / g, and cooling to less than 80 DEG C for later use; 3, preparing EMP: adding measured phosphoric acid into the maleic anhydride modified epoxy resin, reacting for 3 hours at 60-80 DEG C, then adding measured water, reacting for 2 hours at 80-100 DEG C, and hydrolyzing phosphate diester or phosphotriester in EMP into phosphate monoester. The preparation method of maleic anhydride modified epoxy phosphate EMP provided by the invention is high in purification precision, is convenient to manufacture and operate, and is low in cost.