Abstract: In order to reduces the influence of the existing buildings in the surrounding area, the surrounding soil of the new project usually requires the control of the small strain state (0.001%~0.1%). In recent years, the construction scale of underground projects has been continuously expanded, and the mechanical properties of soft clay in the small strain range have received increasing attention. At present, the study on the small-strain mechanical properties of soft soils in triaxial tests is relatively few and lack of the relevant test data. A small strain is developed based on an the LVDT sensors with high-accuracy triaxial apparatus, and K₀ consolidation undrained shear tests on layer ②~⑥ soils are performed of Shanghai soft using the triaxial apparatus. The change of modulus of Shanghai soft soils in the small strain range of 0.001%~20% is obtaived. The shear modulus is normalized by using the initial shear modulus and effective average principal stress, so as to reveal the nonlinear characteristics of Shanghai soft soils as well as the shear stiffness attenuation characteristics with strain, etc. The empirical formula considering stress state, pore ratio and overconsolidation ratio of soils can reasonably describe the initial shear modulus of Shanghai soft soils. The classical model for backbone curve can better fit the attenuation rules of shear modulus of each soil layer.