Based on the theoretical knowledge and engineering practices about vacuum preloading at present,this paper analyzed the change features of parameters including pore water pressure,mud surface settlement,water yield,moisture content and strength of the hydraulic fill mud consolidated by vacuum preloading through test.The test results show that at the earlier stage of vacuum preloading,the surface settlement increases fast,but the pore water pressure increases slowly;in contrast,the surface settlement increases slowly and the pore water pressure increases fast at the later stage.The strength increasing mechanism of hydraulic fill mud consolidated by vacuum preloading was analyzed,and the strength increasing process of hydraulic fill mud was calculated by applying the calculation formula for the strength increase under effective stress consolidation,and combing with monitoring and testing data.The calculation results show that the strength increasing rule calculated with theory corresponds to the strength increasing rule practically measured in the experiment;the strength of hydraulic fill mud is nearly not existed at the earlier stage of vacuum preloading,but increases relatively fast in the middle and later stages of vacuum preloading;The strength increasing trend of soil mass still doesn’t tend to disappear after 7 months’ vacuum preloading.These characteristics about the strength increase of hydraulic fill mud consolidated by vacuum preloading should be brought to the attention in the design and construction of vacuum preloading engineering.The preloading period should be reasonably determined by comprehensively considering the strength increasing rule and the rationality of engineering economy,and the strength index shall be regarded as one of unloading standards.