This study investigates experimentally wave-current interaction and its mechanism and proposes a numerical method to simulate fluid motion under waves and currents in coexisting fields. Hydraulic model experiments were carried out in the open channel installed in a 3-dimensional wave basin to measure the wave height distributions and velocity fields in the channel. The experimental results indicated that the wave height increases initially due to the effects of the current, and then decreases gradually as the wave propagates upstream when its direction opposes the current. The incident wave period affected the decrease in wave height; in particular, a strong current resulted in spilling-type breakers. The results showed that waves also affect the current to weaken its velocity.