The proposed invention discloses an automatic search method for a structural mode signal of a chipless RFID tag. A reference plane, a target to be tested, a reader antenna and a reader are set in space. The target to be tested is located within the reading range of the reader antenna. The distance difference between the tag and the reference plane, to the antenna, is measured by using the phenomenon of the backscattered signals of the tag and the reference plane interfering in space, to obtain the distance between the tag and the antenna. The time period for extracting the antenna mode of the tag may maximize the use of the antenna mode signal, thereby enhancing the accuracy of reading the tag.