It is the first application of 30 cm thick soft mattress of concrete interlocking blocks in deepwater without sheltering in China. Based on the analysis of the stress of soft mattress affected by flow-force, we optimize the geostrip structure, and put forward a proposal that the rang of safety factor shall not be less than 2 times. We also propose the configuration requirements of soft-mattress-laying vessel under adverse geologic conditions, summarize and analyze the application of soft-mattress-laying vessel and the significant impact of the row-flap length on the stress of soft mattress. Based on the analysis and construction control, we effectively improve the quality and the success ratio of soft-mattress laying in deepwater without sheltering.