Abstract: In order to study the deformation characteristics of soil under vacuum preloading when unloading vacuum pressure, compression-redound tests on undisturbed soft clay samples are performed to simulate different vacuum unloading case by normal using the triaxial apparatus and CKC triaxial apparatus. The test results indicate that the deformation caused by vacuum unloading includes elastic volumetric swelling and considerable shear deformation under high deviatoric stress level, and the vacuum unloading can induce the shear failure of ground when the deviatoric stress is high enough, especially in vacuum-surcharge preloading. So, both the volumetric swelling and the shear deformation should be considered in settlement analyzsis of vacuum preloading ground.