Abstract: In order to reveal the 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 were carried out on the basis of the vertical-torsional coupled shear apparatus, and the modulus decline index δ was used to quantify the shear modulus decline characteristics of saturated sand, and the shear strain development characteristics and shear modulus decline characteristics of saturated sand were analysed. The shear strain development characteristics and shear modulus decline characteristics of saturated sand were analysed. The results show that the shear expansion of saturated sand decreases gradually with increasing loading frequency f, and the number of cycles N to reach 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. The shear modulus G gradually decays with the increase of the number of cycles N and the pore pressure ratio ru, and G increases with the increase of f at the same ru and N levels. The effect of loading frequency f on the relationship between δ~ru and δ~N/NL is small, and a new model of δ~ru is established based on the Matasovic model, and a new model of δ~N/NL can be obtained by substituting Seed's one-parameter pore-pressure model into the new model, which can predict the experimental data in this paper better and has better applicability.