On the basis of the principle of effective stress in soil mechanics，the physical meanings of characterization parameters，and the analysis of the boundary conditions for geotechnical test indices，it is concluded that the shear strength of cohesive soil can be calculated using the natural density of the overlying soil layer and the shear strength of the soil layer determined by the direct shear test. This paper also put forward suggestions on the modification method of test indices for the accurate calculation of the shear strength of dredged cohesive soil. The research is of great significance to the classification of rock and soil in dredging engineering.