The invention provides an additive used in hot-dip galvanizing assistant plating agent, belonging to the technical field of metal material plating. The invention is used for solving the problem of assistant plating efficiency. The technical proposal of the invention is that the additive is a de-ionized water solution; the water solution contains cation surface active agent and organic solvent of the long chain alkyl; the content of the long chain alkyl cation surface active agent is 100 to 800 gram per liter; the organic solvent can be ethanol, isopropyl alcohol, acetone or one of nickel and N-dimethyl formamide, with adding content of 30 to 160 gram per liter. The invention has the advantages that the assistant plating agent is made and the made element have a plating face with excellent wettability and compatibility, which can increase the activity and the utilization rate, improve the assistant plating efficiency, prevent plated element from escaping, prevent the plated element which is assistant plated from being secondarily oxidized in air, reduce zinc slag, prevent the zinc liquid from splashing and increase the surface quality of the made element. Furthermore, the invention can be directly added into the assistant agent, and then is evenly mixed, with the character of easy operation. The invention is not limited by the temperature of the assistant plating agent so that the invention can be used under room temperature.