The three-dimensional natural vibration characteristics of water inside a moon pool of an ocean structures are studied. The governing equations are derived based on the linear potential flow theory, and the boundary condition of the total opening bottom suggested by Molin is adopted. A semi-analytical method is used to solve the governing equations, and the natural frequencies and the motion modes are obtained. Two types of motions are studied:(1) the piston motion in the vertical direction, and(2) the sloshing motion of the free surface. The influences of moon pool’s structural parameters on the natural frequencies, and the modal shapes are analyzed. The three-dimensional natural vibration characteristics of water inside a moon pool of an ocean structures are studied. The governing equations are derived based on the linear potential flow theory, and the boundary condition of the total opening bottom suggested by Molin is. two kinds of motions are studied: (1) the piston motion in the vertical direction, and (2) the sloshing motion of the free The influences of moon pool’s structural parameters on the natural frequencies, and the modal shapes are analyzed.