Abstract: Based on observational data of vacuum-surcharge preloading, the paper analyzed the laws of vary with time, which is pore water pressure, excess pore water pressure and relative excess pore water pressure in different depths, and the microcosmic reasons of the changes in the soil pore water pressure produced from the applied vacuum-surcharge pressure. The results indicated: (1)outside of the strengthened area, the changes of pore water pressure are mainly caused by the dropping of the water-table, which are affected less by vacuum degree. (2)inside of the strengthened area, the changes of pore water pressure are influenced by vacuum pressure, the permeability of soil and surcharge pressure. The seepage under the influence of vacuum pressure is the main form of pore water pressure dissipation, and the permeability of soil affects the speed of pore water pressure dissipation. Then the dissipating pore water pressure turns to the effective stress which makes soil grains move to press pore water. Finally, the dissipation of pore water shows pulsation characteristics.