Abstract: The actual engineering problems induced by super low-strength soft clay are paid more and more attention with the rapid development of port and ocean engineering. However, the routine geotechnical experimental method can not test the strength of this kind of soft clay effectively. A RST rheometer used in fluid tests is introduced here to carry out some experiments on the strength and rheological characteristics of the ultra-soft clay. Based on the comprehensive analysis of the experimental results, the quantitative effects of spindles size, strain rate increment and shaft friction are systematically discussed, and an effective experimental approach to test the strength of the ultra-soft clay is offered. Compared with those of the conventional rheometer, the accuracy and the reliability of this apparatus are verified. The new device provides a new way to test the undrained shear strength of ultra-soft clay in submarine engineering.