Based on the first phase project of 12.5 m deepwater channel of the Yangtze River downstream Nanjing,this paper proposes the design ideas of laying technology for the soft mattress of ultra-short baseline (USBL) underwater positioning system according to the deep depth and large flow velocity conditions.Through the system development and system test,we complete the design and apply it to the actual laying work.We also achieve the laying and positioning visualization of soft mattress in deep depth conditions.The working state of the arrangement ship can be adjusted in real time to ensure the accurate laying of soft mattress.