The invention provides a preparation method of heptafluorobutyric acid. The preparation method comprises the following steps: (A) carrying out an electrochemical fluoridation reaction on n-butyric acid, n-butyryl chloride or n-butyryl fluoride with anhydrous hydrogen fluoride, thus obtaining an electrolytic mixed gas, wherein the current density of the electrochemical fluoridation reaction is 0.025 to 0.033 A/cm<2>, and the reaction temperature is 9 to 13 DEG C; a cooling reflux device is arranged in the electrochemical fluoridation reaction, and the temperature of a cooling medium of the cooling reflux device is -45 DEG C; (B) pumping the electrolytic mixed gas obtained in the step (A) into a reaction kettle filled with an acid-binding agent, thus obtaining heptafluorobutyryl fluoride, wherein the acid-binding agent is triethylamine; (C) mixing the heptafluorobutyryl fluoride obtained in the step (B) with water, and hydrolyzing, thus generating a heptafluorobutyric acid water solution; (D) mixing the heptafluorobutyric acid water solution with KOH, and naturally crystallizing until the concentration of the KOH is 40 to 45 percent, thus obtaining potassium heptafluorobutyrate; (E) acidizing the potassium heptafluorobutyrate by using sulfuric acid, thus obtaining the heptafluorobutyric acid.