Seabed soil layer composed of soft sediments, which has a high water content, low bulk density and low shear strength, has great influence on deep-sea engineering devices. Therefore, accurate measurement of the mechanical properties of seabed sediments is a prerequisite for the construction and safe operation of deep-sea projects. In this study, a deep-sea engineering geology in situ test system was developed to measure cone resistance, sleeve friction, pore pressure and shear resistance in seafloor sediments. The system was tested on land, and the feasibility of the system was verified. We conducted sea trials in the South China Sea and acquired datasets from five stations. The data were analyzed using the Eslami-Fellenius soil classification map, and the soil classification of the site was obtained. The obtained values of cone resistance, sleeve friction, pore pressure and shear resistance can provide mechanical data support for deep-sea engineering in this area.