The Pastor-Zienkiewicz-Mark III (PZIII) constitutive model, which can be used to describe the nonlinear behavior of sandy seabed soil, is implemented into a well-validated computer code, FSSI-CAS 2D, for analyzing the wave-seabed-structure interaction. The characteristics of wave-induced liquefaction in offshore loosely-deposited sandy seabed is quantitatively investigated. The analysis results indicate that the developed coupled numerical model FSSI-CAS 2D is capable of capturing a series of the properties of wave-induced cumulative liquefaction in loose seabed, as well as its wave-induced dynamics. The results also show that the wave-induced liquefaction in seabed is progressive. That is, wave-induced liquefaction initiates at the surface of seabed, and propagates downward under long-term wave loading.