The measurement error of dual-frequency echosounder and its influence on navigable depth survey during period of fluid mud is discussed based on the field measured data obtained by Odem Echotrac MKIII echosounder and Stema Densitune meter. The results are as follows: 1) High-frequency echosounder record may include part of the fluid mud layer and low-frequency echosounder record equivalent to the consolidated bed under the situation of initial stage fluid mud. 2) High-frequency echosounder record equivalent to the upper limit of the fluid mud and low-frequency echosounder record may locate in the fluid mud layer which is higher than the consolidated bed under the situation of typical fluid mud. 3) High-frequency echosounder record equivalent to the upper limit of the fluid mud and low-frequency echosounder record mostly locate at the fluid mud density mutation which is nearest the upper limit of the fluid mud under the situation of consolidating stage fluid mud. 4)Low-frequency echosounder record is no larger than the navigable depth while fluid mud exists in the navigation channel under the situation of consolidating stage fluid mud or sometimes existence of typical fluid mud, which may waste part of the real navigable depth.