Abstract: Some concepts of shear strength and shear strength parameter, undrained shear strength and unconsolidated- undrained triaxial test are wrong in some soil mechanics textbooks and geotechnical engineering codes. The differences between the shear strength and the shear strength parameter are firstly discussed. Then the shear strength parameters determined by means of the consolidated undrained triaxial tests and the undrained shear strength determined by unconsolidated-undrained triaxial tests are analyzed. The differences between the undrained shear strength and the shear strength parameter of the soft clay are discussed. With the discussion about determination of the shear strength in the soft clay ground, the differences between the undrained shear strength and the shear strength parameter of the soft clay are discussed again. Finally, the stability analysis method, calculation parameters, determination method of calculation parameters and safety factor are coincided in the stability analysis of geotechnical engineering.