Abstract: The study on development of the shear strength of soft clay is very significant for the use of vacuum preloading method. The vane shear tests are carried out before, during and after the vacuum preloading. It is shown that the shear strength increases rapidly in the early stage of the vacuum preloading, but slowly in the late stage. The growth mechanism of the shear strength under the vacuum preloading is analyzed. The law of the shear strength growth under the vacuum preloading is verified by using the shear strength calculation equation based on the effective consolidation stress method and parameters of soft clay. The process is simulated by use of FEM, showing that the shear strength of soft clay in the early stage under the vacuum preloading increases faster than that in the late stage, and that the shear strength of soft clay increases not linearly but non- linearly, and that the shear strength of soft clay increases unlimitedly. The characteristics of the shear strength growth under the vacuum preloading should be paid attention to sufficiently in the engineering design and construction using the vacuum preloading method or vacuum-combined surcharge preloading method.