The invention discloses a plating auxiliary for plating an alloy coating with high aluminum content on steel by using a solvent method. According to a formula, the plating auxiliary comprises the concrete components according to the content (g/L): 20-60g/L of potassium borohydride, 1-10g/L sodium hydroxide, 20-40g/L of calcium chloride, 10-40g/L of potassium fluoride, 3-20g/L of lithium chloride, 2-12g/L of sodium fluoroaluminate, 5-30g/L of potassium fluotitanate, 1-10g/L of nonionic surfactant and the balance of water. The plating auxiliary disclosed by the invention is simple in production process and capable of forming a uniform salt film on a piece to be plated. During hot plating, the plating auxiliary is not reacted with aluminum in a zinc solution, and sodium fluoroaluminate in the plating auxiliary can be used for cleaning an alumina film on the surface of the zinc solution, so that the plating leakage phenomenon is avoided, and the surface of a coating is bright.