Abstract: In order to reveal shear strain development characteristics and shear modulus decline characteristics of saturated sand under different loading frequencies, undrained cyclic torsional shear tests of saturated sand with different relative densities under different loading frequencies are carried out on the basis of vertical-torsional coupled shear apparatus, and the modulus decline index δ is used to quantify shear modulus decline characteristics of saturated sand, and shear strain development characteristics and shear modulus decline characteristics of saturated sand are analysed. The results show that shear expansion of saturated sand decreases gradually with increasing loading frequency f, and the number of cycles N_L of initial liquefaction increases gradually; and the increase of f inhibits the development of strain in saturated sand. δ at similar shear strain levels decreases with increasing f, and a modulus decay model based on shear strain is proposed to predict δ. Shear modulus G gradually decays with the increase of the number of cycles N and pore pressure ratio r_u, and G increase with the increase of f at the same r_u and N level. The effect of loading frequency f on the relationship between δ-r_u and δ-N/N_L is small, and a new model of δ-r_u is established based on Matasovic model, and a new model of δ-N/N_L can be obtained by substituting one-parameter pore-pressure seed model into the new model, which can predict the experimental data better in this paper and has a better applicability.