Abstract: Foundation soils of offshore-island transportation infrastructures are continuously subjected to various dynamic loads. The dynamic shear characteristics of the foundation soil under the small-to-medium strain range are important to the evaluation of the infrastructure serviceability. A series of resonant column tests are conducted on clay samples taken from a land reclamation site in the Zhoushan sea area, to investigate the effects of soil state and shear strain amplitude on the shear modulus. The test results indicate that the small-strain shear modulus is positively correlated with effective stress and negatively correlated with void ratio. As the shear strain enters the medium-strain range, the shear modulus gradually reduces with increasing strain amplitude in a nonlinear trend. The effective stress level has certain impacts on the strain-dependent shear modulus degradation, but the void ratio does not affect the pattern. Based on the test results, empirical formulas are proposed to characterize the small-strain shear modulus and the strain-dependent behavior of shear modulus. The experimental data from this study not only enrich the database for the dynamic properties of clay but also provide a basis for the serviceability evaluation of offshore-island transportation infrastructure in the Zhoushan sea area.