The invention provides a method for preparing mint-based carboxylic acid in a process of synthesizing N-Ethyl-p-menthane-3-carboxamide. Metal magnesium chips, tetrahydrofuran and methylbenzene are added in a stainless steel pressure reaction kettle with the volume of 1L, which is internally provided with a mixer, a thermometer and a reflux condensation device; the kettle is heated to be about 80 DEG C; under a mixing condition, the mint-based chlorine made of natural menthol is dropped slowly; carbon dioxide is introduced in the pressure reaction kettle under a sealed state; the flow rate of the carbon dioxide is adjusted, the pressure of the reaction kettle is kept at about 0.5MPa; the reaction time is kept about 10 hours; the reactants are acidified and washed by 30% of hydrochloric acid; the mint-based carboxylic acid is obtained after recovering the solvent; 85g of mint-based carboxylic acid is obtained by weighing; by gas chromatography, the content of the mint-based carboxylic acid is 87.5% (levo form); the mint-based carboxylic acid reacts with the thionyl chloride to obtain the mint-based acyl chloride; the mint-based acyl chloride reacts with ethylamine solution (70%); the N-Ethyl-p-menthane-3-carboxamide (WS-3) cool flavorings are obtained by rectifying, crystallizing, pumping and filtrating and drying; and 52g of N-Ethyl-p-menthane-3-carboxamide (WS-3) is obtained by weighing.