In view of the underwater detection problem of gravity wharf,we probe into the application of three-dimensional imaging sonar.We analyze the working principle and instruments,and study the station layout,parameter settings and data processing of three-dimensional imaging sonar in gravity wharf engineering.By point cloud image,we realize the qualitative detection of the underwater obstacle,caisson placing quality and caving,and quantitative detection of the component size,caisson fault and caisson clearance.Moreover,the underwater topographic maps are drawn.The article can provide reference for other under water enineering inspection.Applying this technology to the gravity wharf in the detection of underwater structure,it is implemented of the gravity wharf structure size measurement,underwater obstacle recognition,caisson placin quality testing and caving detection.