The invention provides a novel plating auxiliary for single hot-dipping of a Galfan alloy in a solvent method and a preparation method thereof. According to the plating auxiliary, per liter of the plating auxiliary is prepared from the following components of 100-150 g/L of ZnCl2, 50-100 g/L of ZnBr, 10-20 g/L of SnCl, 8-12 g/L of KF, 0-0.5 g/L of bismuth chloride, 0.5-2 g/L of sodium borohydride,0.1-0.3 g/L of hydrochloric acid, 5-15 g/L of active alkali metal chloride salt and the balance water. The plating auxiliary has the advantages that the zinc bromide, the sodium borohydride and the potassium fluoride are introduced into the plating auxiliary, so that a surfactant and rare-earth chloride which are expensive in price are avoided, the application range is wider, almost all steel products are covered, and therefore the problems of serious skip plating, air holes, list edges, slag inclusion and the like in the Galfan alloy hot-dipping process in the prior art can be effectively solved; and the plating auxiliary can play a good plating assisting role when being used at room temperature, the production cost is greatly saved, the components of the plating auxiliary are stable, the cost is low, and industrial popularization is facilitated.