Due to the restriction of work environment of water conservancy engineering underwater, the conventional hidden danger detection methods are difficult to achieve accurate detection of safety hidden danger of underwater engineering.On the basis of discussion on common used underwater detection technology (equipment) and application development, this article introduced the basic function of three-dimensional real time sonar imaging system,and practiced the technology in Yanshiling Reservoir and Kanmen fishing port breakwater.The result shows that the system can quickly and accurately detect the underwater safety hidden trouble to water conservancy project such as reservoir, seawall (dike),and can generate continuous and clear three-dimensional image,and has stronger applicability and can provide the data reference for the construction,safety operation and management of water conservancy.