Abstract: Dynamic pore water pressure would occur due to the wave-current acts on the seabed. If it cannot be eliminated in time,cumulative pore water pressure would grow in the seabed. The hydraulic gradient caused by the difference of pore water pressure between two adjacent points can produce seepage force,and then cause water flow. The seabed surface is a drainage interface,so an upward seepage force would form in the seabed and act on the sediment particles. It leads to the sediment transport and movement to the seabed surface,thus forming a certain range of coarse-grained layer. In this paper,numerical simulation is used to study the cumulative pore water pressure under different velocity. The influence of the hard shell layer on the cumulative pore water pressure is analyzed. The calculation method of critical scour depth of seabed established by Wang et al.(2014) is used to analyze the final depth of hard shell layer under different velocity. The results show that when the direction of wave and current is the same,it would promote the cumulative pore water pressure. The greater the velocity is,the greater the cumulative pore water pressure is. The opposite direction would inhibit the cumulative pore water pressure. The surface hard shell layer can significantly promote the dissipation of cumulative pore pressure. When the velocity U₀=0m ·s^-1,the thickness of the hard shell layer increases from 1m to 3m,and the depth of the extreme point decreases by 1.38m. The seepage force caused by the cumulative pore water pressure has a significant effect on the seabed sediment movement. When the velocity at both U₀=0m ·s^-1 and U₀=1m ·s^-1,the depth of the sediment incipient motion is 1.5m. If the direction of wave and current is the same,it would produce a larger ΔP/ΔL value to drive the coarse sediment particles to the surface of the seabed,but has little effect on the maximum depth of the sediment incipient motion.