Abstract: The measurement and analysis of pore water pressure is significant for the studies on under watervacuum preloading method.In order to study the variation of pore water pressure,the full-scale field tests were performed.It was indicated that the variation of pore water pressure was influenced by water pressures on the sealing membrane and negative pressures in sand cushion.The variation of stress in soil was equal to the difference between the values of pore pressure before and during vacuum preloading,indicating that water pressures on the sealing membrane could act as preloading loads.When the depth of water on membrane increased,the positive pore pressure would be induced in soil which,and it similar to the surcharge load.The distribution of negative pore pressure in soil was not uniform,causing differential settlement and strength increase.