The invention provides an ammonium-salt-free plating assistant agent used for hot dip galvanizing and technical methods for preparation and use thereof, belonging to the technical field of the surfaceplating technique of steel and iron materials. The technical proposal is as follows: the aqueous solution of the plating assistant agent has the following components of zinc chloride, sodium chloride , a nonionic surface active agent (cocinic acid alkanolamide, fatty acid alkanolamide or alkanolamide phosphate ester) and a perfluoro-carbon type surface active agent with the respective proportion in sequence of 80 to 350g/L, 30 to 100g/L, 2 to 4g/L and 0.1 to 2g/L, and the balance of water. When the plating assistant agent is used for hot dip plating: the steel and iron product after oil and dust removing is dipped into the ammonium-salt-free plating assistant agent for plating assistant treatment firstly, the temperature is controlled to be in the range of 40 to 80 DEG C, then drying or baking is carried out at the temperature of 40 to 140 DEG C so as to lead the surface of the product to be adhered with a layer of even plating assistant film, and then the obtained product is dipped into molten zinc for hot dip plating. The invention has the advantages that the plating assistant agent is not added with ammonium salt, thus being capable of greatly reducing the generation of dusts when in hot dip plating, reducing pollution to environment and improving working environment.