To investigate the transient liquefaction stability of dense seabed soil beneath the offshore bridge under extreme wave loads, a finite element numerical model for simulating dynamic response of seabed is developed and solved by RANS equation and Biot equation. Accuracy of the proposed model is verified by experimental data from literatures. The distribution of the wave pressure field is applied to analyze the effects of the wave characteristics and the submerged depth on the transient liquefaction stability of dense seabed. Results of numerical simulation indicate that the submerged girder can significantly affect the wave pressure field, and the amplitude of the liquefied depth in front of the girder is greater than that in rear of the girder. The largest liquefied depth is located within the range of 1/10-1/8 distant from the front of the box girder. The amplitude of the liquefied depth increases with the increment of both wave height and wave period. The seabed in front of the box girder is the easiest to be liquefied when the box girder is just submerged while the seabed gets more stable with the increment of the submerged depth in rear of the girder. The results of the study can provide a reference for the safety analysis of cross-sea bridges.