During the sub-sea tunnelling through a seawall, the soil adjacent to the tunnel may simultaneously bear tunneling-induced stress relief and wave-induced cyclic shearing. To investigate the coupling effects of stress relief and repeated loading on the mechanical behaviors of soft clay, a series of experiments were carried out using a hollow cylinder apparatus, which imposed undrained cyclic shearing to the soil samples under a reducing confining stress, followed by undrained shearing. The cyclic stresses simulated in the tests were estimated using elastic solution for a seabed subjected to the wave loadings. The test results show that the variation of each stress component under the coupled loading follows a sine or cosine relationship. The coupled loading plays a more significant role in affecting the radial strain (εr) and hoop strain (εθ) than the vertical strain (εr) and the torsional shear strain (εzθ) of the soft clay. The shear modulus of the soil experiences more degradation under the coupled loading than that under the single action of cyclic shearing. The pore pressure of the soil reduces under the disturbance of shield construction and begins to increase slowly under the wave loading after construction. At a given cyclic magnitude, the reduction of soil strength significantly depends on the magnitude of stress relief. A larger stress relief leads to a more reduction in the post-cyclic shear strength of the soil. Based on the experimental findings presented above, it is suggested that coupling effect between the cyclic shearing and the stress relief should be taken into account while designing sub-sea tunneling through a seawall.