Aiming at the problem of inadequate detection method of the plastic drainage belt in the deepwater area of the Yangtze estuary,we propose a new method of underwater acoustic imaging detection,by which we realize the visualization and measurement of underwater drainage belt adopting the high-resolution side-scan sonar system and DIDSON (dual-frequency identification sonar),obtain the clear image of the underwater drainage belt,and realize the quantitative detection of the plastic drainage belt by the DIDSON using a fixed-point full loop imaging mode.The new method makes up the insufficiency of the existing detection means,and can be used as an important supplement to the existing detection methods.