Large-scale cofferdams have been widely used to create a relatively safe construction environment for the sea-spanning bridges. The study on the wave/current-induced seabed stability around a cofferdam has become one of key issues to guide the sea-spanning bridge structural design and ensure structural safety. Unlike the previous studies, the wave–current interactions are considered. The hydrodynamic response and seabed response around the dumbbell cofferdam under combined wave and current loading are studied. The numerical results conclude that: (1) The current has a significant effect to the hydrodynamic response around the dumbbell cofferdam. The dynamic force acting on the cofferdam gets a larger value under the combined action of wave and the following current compared with that under the wave alone. (2) The dumbbell cofferdam can obviously affect the influence of the current on the pore-water pressures. In the back of the cofferdam, an obvious mutation point appears in the influence trend of the current on the pore-water pressures at the embedded depth. (3) The liquefaction zone can be expanded as the following current is superimposed on the wave load. The following current is more unfavorable to the wave-induced seabed stability around the cofferdam.