Applying the simplified model and changing the permeability coefficient of geotextile at model openings, this paper simulates the actual changes of underground water level at treatment interior area during soft ground treatment by vacuum preloading based on the laboratory test. The test results show that the underground water level declines during vacuum preloading. Fitting the curve of H-(-lgK), it is concluded that the less the soil's permeability coefficient is, the greater the water level declines. The test reveals that we need to consider the water supply at the reinforcement area by surrounding groundwater and draw up the new standards of seal design to the surrounding reinforced area.