Abstract: The quality of the soft soil includes low strength and high compressibility, and the deformation shows obviously different nonlinear characteristics under different stress states and paths. It is of great significance to study the in-situ deformation characteristics of soft soil for controlling the deformation in various underground projects. In this study, such deformation characteristics are well described from two aspects of loading and unloading through PMT in Shanghai area. The relationship between shear modulus G and shear strain γ is obtained by the pressuremeter loading tests combined with laboratory small-strain tests. At the same time, the internal relationship between modulus ratio E_v and unloading ratio R is studied under different maximal unloading pressures and unloading levels by the pressuremeter unloading tests. The results reveal the stiffness degradation law for loading tests and the key factors determining the stiffness characteristics for unloading tests, which will provide the relevant reference for controlling soil deformation in the development of underground space.