Abstract: With regard to some phenomena being difficult to explain by the existing theories in vacuum preloading,vacuum preloading model tests on sand and clay with rigid caps and flexible caps were carried out,and those on sand with rigid caps were also performed by use of continuous and discontinuous drainage passages respectively.Based on the analysis of test results,it was put forward that the effect of vaporization and capillarity of void water was very small,and a new important drainage by expansion of solution gas which could drive void water effectively was given in addition to the drainage by consolidation in vacuum preloading.The drainage by expansion of solution gas in vacuum preloading was proved to be in existence theoretically,and some phenomena in practice were explained more reasonably.