In view of the lack of detecting measures for deepwater laying of the soft mattress,this paper presents a new detecting technology,which integrates water-depth grid of multi-beam bathymetric system and sound & image data generated by the side-scan sonar,and uses the ultra-short baseline measuring system to verify the determined mattress coordinates after getting the mattress’s sinking and laying position.This technology has been applied to Phase II of 12.5 m waterway project of the Yangtze River downstream from Nanjing and improved the detection efficiency and quality of the project.