The vacuum and siphon dewatering tests were performed to investigate the dewatering behavior of the high moisture content ultra-soft clay, respectively. All the tests are conducted in the small-scale tanks with plastic drainage board installed horizontally at the bottom. The influence of the magnitude of the negative dewatering pressures on the dewatering behavior of the dredged slurry was investigated. The test results show that the initial dewatering efficiency shows no significant difference when the slurry exists in the slurry-like state. Thereafter the dewatering efficiency increases with the magnitude of the applied dewatering pressure. The test results also show that the corresponding compression indexes in the pressure ranges from 0 to 30 kPa and 30 to 80 kPa are 0.33, 0.3, respectively.