A laboratory device to measure horizontal displacement of soil around a region treated by vacuum preloading includes a box injected with dredger fill. A vertical drainage board and a vacuum pumping tube are buried within soil in the treatment region in connection with a vacuum degree detector and a vacuum pump. A seal membrane covers soil in the treatment region. A loading device is above the seal membrane. Transverse tubes are inserted into a side of the box and in surrounding soil of the treatment region. An annular sensing source, which moves together with the soil and which is movable along the transverse tubes, sleeved on the transverse tubes. Each transverse tube is provided with a sensing instrument, and the sensing instrument is connected with a sensor is inserted into the transverse tube. The different positions of the sensing source at different moments of time are determined.