Abstract: Based on the analysis of the results of model test,the mechanism of consolidation of soil improved with the underwater vacuum preloading method was proposed,and it provided a theoretical basis for the implementation of the method.The preloading load imposed by underwater vacuum was equal to the difference between the pore water pressure before the implementation of the underwater vacuum preloading method and the pore water pressure after the implementation of the method(when the soil was fully consolidated).When the pore water pressure in the sand cushion beneath the sealing membrane was negative,all the water retained on the sealing membrane might act as the preloading loads.