Abstract: Wave action leads to complex pore-water-soil-skeleton coupling effects of seabed soils, that is, the soils are subjected to three-dimensional coupled shear action of normal stress difference and shear stress, and the transient excess pore water pressure increases in two modes of oscillation and accumulation, resulting in cyclic softening of soils. Most of the already-established Masing nonlinear dynamic constitutive models have failed to fully consider the growth mode of the excess pore water pressure and the cyclic softening effects of the soil skeleton. In this study, the Davidenkov skeleton curve is extended to the three-dimensional stress space, and the stress-strain hysteresis curve of soils is constructed by the "extended Masing" rule. Based on the Biot dynamic consolidation theory, the shear volume strain coupling model proposed by Byrne is introduced into the fluid continuity equation as the source term of the growth of the cumulative excess pore pressure, and a nonlinear dynamic constitutive model for soils is established to describe the pore-water-soil-skeleton coupling effects. Based on the FLAC^3D platform, the secondary development of the constitutive model is realized. The calculation accuracy of the program is validated by comparing with the theoretical solution of the model. The feasibility and the rationality of the proposed nonlinear dynamic constitutive model to deal with such problems are verified by comparing with the soil element cyclic torsional shear tests. It provides an effective means for the study on marine geotechnical engineering using the FLAC^3D.