The invention provides a preparation method of a modified bipolar plate for a vanadium flow battery. The method includes uniform raw material component formation, surface conductive agent spraying, twice hot-pressing, and constant-temperature cooling. The method provided by the invention fully hot melts a conductive coating and a base layer, which greatly reduces the disadvantages that a prepared bipolar plate has a layered structure, and solves the increased surface resistance and the structural change after overcharging and long-time operation. The prepared bipolar plate has the advantages of low surface resistance, good mechanical properties, good oxidation resistance and corrosion resistance.