The phenomena in the vicinity of seabed surface such as sediment movement are closely related to the wave-induced seepage in the seabed. In this study, the mechanism of seabed scour is investigated, and the estimation of critical conditions for the scouring due to wave-induced seepage force is discussed in detail. Results show that the incipient motion of sediment and the scouring under wave actions are a process that goes progressively downward from seabed surface until reaching a critical scour depth eventually. The critical incipient shear stress of sediment can be reduced and the incipient motion of sediment can certainly be promoted due to the wave-induced seepage force. The wave-induced seepage force is considered as an important driving factor in the estimation of critical conditions for the scouring. The seepage force is supplemented to the traditional formula for estimating the incipient motion of sediment, and a new method for the estimation of critical scour depth due to wave-induced seepage can thus be derived. Two case studies are performed. The "fluidization" phenomenon in the laboratory flume and the genetic mechanism of the submarine geological hazards such as the gully of the silt flow in the Yellow River delta are well explained. The effectiveness of the new method for estimating and assessing seabed scouring has been preliminary tested.