This paper analyzes the process of drainage and load variation of dredged mud during soft ground treatment by the vacuum preloading, discusses theoretically the variation law of ground’s shear strength along the depth direction after ground treatment under the three conditions: vacuum preloading with no sand cushion;vacuum preloading with sand cushion; laying sand cushion after vacuum preloading with no sand cushion. It is found that the sand cushion plays an important role in the process of soft soil ground treatment with vacuum preloading method.