Abstract: In view of the difficulty in testing the strength of submarine soft clay and the lack of test accuracy, a new type of full-flow penetrometer is developed for testing the strength of soils with the high water content and low strength, and its test validity is verified. On this basis, a series of full-flow strength tests on the undisturbed samples, some typical soft clay from the northern slope of the South China Sea, are carried out. The range of values of the initial resistance coefficient N and the remolded resistance coefficient N_rem in the tests are analyzed, and the distribution characteristics and trends of the undrained shear strength of soft clay in the study area along the seabed depth direction are presented.Then the structural characteristics of soils are discussed combined with the macro structure sensitivity measured by the full-flow strength tests and the micro pore area ratio measured by the scanning electron microscope tests. Finally, a normalized undrained shear strength model suitable for soils in the study area is proposed based on the relationship between the remolded shear strength and water content / liquid limit. This research will provide a reference for the development of seafloor energy resources, the ocean engineering foundation design and the prediction of geological hazards in the northern slope areas of the South China Sea.