Abstract: Four series of cyclic triaxial tests with different loading forms are conducted on saturated coral sand from Nansha Islands. The relationships between shear modulus ratio G_1st/G₀ of the first loading cycle and shear strain amplitude \gamma_a under different relative densities D_r and effective confining pressures \sigma_0^' are studied. D_r shows no influence on G_1st/G₀, and G_1st/G₀ increases with the increasing \sigma_0^'. When γ_a is larger than 3×10^-4, the G_1st/G₀ -\gamma_acurves obtained from the strain-controlled multistage loading tests (UMγ-CTX) are higher than those obtained from strain-controlled single-stage loading tests (USγ-CTX) because of the change of the soil structure and the relative density during the reconsolidation process of UMγ-CTX. A new damage parameter P_d is proposed based on the elastic strain energy theory, and the relationships between shear modulus ratio G/G₀ and damage parameter P_d under different loading forms are studied. The (1-G/G₀) - P_d curves are almost linear in the log-log coordinate, and the slopes are dependent on the shear strain amplitude \gamma_a and the maximum shear strain amplitude \gamma_a.max. Based on the G_1st/G₀ -\gamma_aand G/G₀ - P_d relationships, a new G/G₀ prediction model is proposed to predict the shear modulus of coral sand considering the effect of coupled shear strain amplitude and damage state.