The invention relates to the technical field of fluorochemical preparation, particularly a preparation method of perfluoroheptanoic acid. The method comprises the following steps: oxidizing dodecafluoroheptanol to obtain dodecafluoroheptanoic acid, carrying out reaction on the dodecafluoroheptanoic acid and thionyl chloride to obtain dodecafluoroheptanoyl chloride, carrying out fluorination on the dodecafluoroheptanoyl chloride to obtain perfluoroheptanoyl chloride, and hydrolyzing to obtain the perfluoroheptanoic acid. The technical process of preparing perfluoroheptanoic acid from dodecafluoroheptanol has the advantages of low energy consumption, no pollution and high yield (up to 75% or above), is easy to operate, and has higher competitive edges than the existing perfluoroheptanoic acid production technique.