Abstract: Hard crust, usually called monolithic sand, exists in the shallow stratum at the Yellow River subaqueous delta. There are two possible explanation mechanics for the formation of monolithic sand: one explanation is that the monolithic sand formats during the sediments depositing process, while the other is that hydrodynamic forces such as waves make the deposited sediments transform into monolithic sand. In the paper, we exerted cycle vibration loading on the inter-tidal mudflat and measured the sediment strength after the test; the data shows that after some stewing time the sediment strength is higher than that before vibration. And in the flume experiments on a silt bed in lab, it is found that some low-strength area vibrates with wave shearing action, which leads to a moisture content depression and a density increase. Our analysis indicates that the monolithic sand at the Yellow River subaqueous delta comes into being in this way: while large waves come, the shallow stratum sediments strength gets depressing until it comes below the shear stress exerted by waves, and then the shallow stratum sediments begin to oscillate with waves, when the waves get smaller or perish, the sediments get stewed and the density and strength increase, and then the monolithic sand shows up.