Abstract: The sand-clay mixtures are made by mixing sand and marine clay with different dry weight proportions homogeneously. Undrained triaxial tests are conducted to study the effects of sand content variation and confining pressure on the initial shear modulus and stress-strain relationships of the mixtures. Mechanisms are analyzed. Tests results show that there is no obvious relationship between the initial shear modulus and the sand content variation when sand content is less than 50%.For sand-clay mixtures with sand content ranging from 50% to 80%,the initial shear modulus increases linearly with the growing sand content. The sand content parameter should be introduced into the initial shear modulus so that the modified Duncan-Chang model can be used to imitate the stress-strain relationships of the sand-clay mixtures with sand content less than 80%effectively. When the sand percentage is over than 80%,the characteristic of strain softening occurs. The initial shear modulus is kept steady gradually. The peak strain becomes smaller. The softening model is useful to describe the stress-strain relationships of sand-clay mixtures in this case. The confining pressure has a visible effect on the stress-strain relationship of sand-clay mixtures. As the confining pressure increases, the initial shear modulus shows a linear growing trend in double logarithmic coordinate.