Abstract: Based on the potential flow theory of liquid and vibration equation of cantilever beam, the free vibration problem of cantilever beam-liquid-cantilever beam coupled system is analyzed in this paper. The analysis shows that both the beam-liquid interaction and the beam beam interaction are present. The latter interaction gives rise to a new dynamic property in the coupled system. Two same order dry modes of substructures(beams 1,2) correspond to two same-order wet modes of the coupled system respectively. For the lower wet frequency, the two beams are in the opposite phase. For the higher wet frequency, the two beams are in the same phase.