The device utilizes the reflection characteristics of the elastic wave, and obtains the elastic wave propagation time by recognizing the reflected signal at the bottom end of the column, thereby calculating the length and depth of the column. The column is a hollow thin-walled structure with a large surface area, and the elastic wave induced by the excitation is not only longitudinal but propagates along the column wall and returns along the column wall. The induced elastic wave is complex, and the propagation path cannot be completely controlled. The buried column is closely combined with geotechnical and concrete. There are different impedance differences, and there are many different reflection signals. Even such reflection signals are stronger than the column. The bottom end reflection signal, together with the free vibration that is not well suppressed due to the installation of the excitation device is not well, which causes interference to the automatic recognition of the reflected signal, and appropriately sets the preset length interval, which is beneficial for filtering out some non-bottom portions. The end reflection signal interferes. In addition, the instrument also uses half-wave movement, amplification processing and other techniques to enhance the recognition of the reflected signal at the bottom of the column.