In consideration of the size effect of the small-scale cylinder and the obvious lateral friction effect of the square box in a dredged sludge vacuum drainage laboratory test, an instrument for the dredged sludge vacuum drainage laboratory test was developed, with a large-diameter cylinder used as the test container. The instrument consists of seven parts, including the model box, drainage system, pressure system, vacuum pumping system, valve controlling system, tail water collection system, and measurement system. Three groups of comparative experiments for instrument testing under three vacuum conditions show that the test results accord with the basic law of vacuum drainage, and also confirm the existence of the optimal vacuum degree.