The kinetic characteristics of floating mud in the estuarine bottom play an important role on the deposition process in harbors or channels and the maintenance of estuaries. And the study on them has become one of the focuses of coastal researchers. In this paper, the significance of the study on sediment density flow in estuaries is summarized firstly, the differences and the conditions of the functions applied in the rheological relation of the mud by many investigators are analyzed. Then, according to the actual needs of the research problem in this paper, a two-layer fluid model with the upper-layer fluid setting as a Newtonian one and the lower-layer fluid being simplified as Non-Newtonian, was established under the wave motion. Additionally, as one of the important functions, the power law is adopted as the constitutive relation of the fluid. The flow field of the density current is then studied, including the velocities, pressures of the two layers fluids, the wave amplitude ratio of the interface to the free surface and so on. Additionally, the influences of the mud density, the circular frequency and the power-law exponent on the flow field are discussed in details. It is found that the velocities of the both layers are consistent with each other at the interface, while the pressure has a sudden change due to the existence of the velocity gradient along z-axis. In the lower layer, there is an extreme point in the curve of horizontal velocity amplitude. The differences of the pressures between the two layers become greater, with the increase of the circular frequency or the decrease of the mud density as well as the power-law exponent. Finally, the comparisons between the calculated results and the experimental data prove the validity of the model.