This paper examines the soil characteristic and liquefaction degree of hard shell seams at the subaqueous delta of the Yellow River. Four typical areas were chosen for measuring, testing and studying. The hard shell seam strength was measured by in-situ Proctor penetrometer. Undisturbed soil sample of 1 m long was excavated for laboratory soil property test. Sledge hammers were dropped down freely to hit the load board to simulate the wave force acted on the hard shell seam. The excess pore pressure and strength variation were observed. The liquefaction depth of the hard shell seams at four study areas under different wave storms were calculated by theory. From the studies, some conclusions were gotten: (1) the hard shell seams are in overconsolidated state, and overconsolidation ratio decreases with depth. The strength of the hard shell seams at Dawangbei and Diaokou are twice of those at Xintan and Guangli ports. The strength variation coefficient near the estuary is bigger than those far from estuary. (2) According to the pore pressure variation with vibration times, the pore pressure variation can be divided into four stages, initial stage, growth stage, stabilization stage and attenuation stage. During vibration, the surface soil liquefied, however the deeper soil did not liquefy. (3) The liquefaction depth of the hard shell seam at Da Wang Bei is 7 to 11 cm under 6th to 10th storms, and they reach 32 to 42cm at Xin Tan and Guang Li ports. The liquefaction depth of the hard shell seam with high strength was small, however the seam with low strength liquefied deeper, thus the difference of liquefaction depth caused the non-uniformity of seabed.