Reagents of the kit comprise the first reagent composed of isooctane, the second reagent formed by dissolving sodium acetate in distilled water, then adding pyridine and mixing the constituents uniformly and the third reagent formed by dissolving copper acetate in distilled water, then adding pyridine and mixing the constituents uniformly. The method comprises the steps that free fatty acid in a sample is extracted through the isooctane, then the free fatty acid reacts with copper salt to generate copper soap under the weak acidic condition, and the content of free fatty acid in the reaction sample at the light absorption value of 715 nm is measured.