An improved variable-water head permeameter is used to simulate the water migration in dredged sludge during the dewatering process. By determining the permeability coefficient and the filtration constant, the seepage volume and the filtrate volume, the water content and the porosity of the mud cake, the influences of different vacuum loading methods (with different durations and loading gradients) are examined on the filtration and seepage of the dredged sludge during the dewatering process under the vacuum load level of 100 kPa. It is found that the filtration and seepage occur simultaneously under the vacuum loading condition. During the 1 h loading, the filtration prevails for 40 minutes, whereas the seepage dominates in the following 20 min. When the vacuum loading gradient increases from 1 time/ 1 h (gradient: 100 kPa) to 5 times/ 1h (gradient: 20 kPa), as the vacuum loading gradient decreases, the permeability coefficient and filtration constant increase, and the dehydration of dredged sludge is significant. The smaller the vacuum loading in the early stage is, the more effective the dehydration is.