The invention relates to waterborne polyurethane polyamide resin emulsion for a metal surface coating and a preparation method of the emulsion. The emulsion is prepared from the following components in parts by weight: 70-85 parts of polyamide resin polyalcohol, 15-30 parts of polytetrahydrofuran, 5-25 parts of amino silicon oil, 65-85 parts of isophorone diisocyanate, 0.5 part of organic bismuth, 5-15 parts of 1,4-butanediol, 10-15 parts of dimethylolpropionic acid, 5-10 parts of diethylenetriamine and a proper amount of a solvent. The preparation method comprises the following steps: mixing polyamide resin polyalcohol, polytetrahydrofuran, amino silicon oil, isophorone diisocyanate and organic bismuth, heating for reaction, adding 1,4-butanediol and dimethylolpropionic acid for reaction, adding the solvent during the reaction to control the viscosity, cooling, emulsifying, and adding diethylenetriamine for continuous reaction. The emulsion can be applied to a metal surface, is equivalent to an overseas similar product in performance, is much cheaper than the overseas similar product sold in China, has relatively good market prospects, and can bring relatively good economic benefits to production enterprises.