Introducing the decoupling coating is an effective way to reduce sound radiation from underwater structures. In order to investigate the decoupling mechanism of a viscoelastic coating layer with horizontal cylindrical cavities, such a coating layer is approximated to a homogeneous layer with equivalent material properties, and a theoretical model is also developed to predict the sound radiation from a finite plate with such a decoupling coating layer. #br#The validity of the theoretical model is confirmed by comparison with the finite element method; and the decoupling mechanism of the coating layer is discussed. Numerical analysis shows that: (1) The energy flow across the interface between the plate and coating layer is mainly conveyed by longitudinal waves. (2) At a low frequency, the coating layer has nearly no decoupling effect. (3) In contrast with a homogeneous coating layer, the coating layer with horizontal cavities can greatly enhance the mechanical impedance in the mid- and high-frequency areas; hence the mean square velocity is effectively suppressed in the same area. (4) Compared with the homogeneous coating layer, the coating layer with horizontal cavities has a larger impedance mismatch with water, thus it exhibits great vibration transmission loss. Therefore, in general, the coating layer with horizontal cylindrical cavities has a better decoupling performance than the homogeneous coating layer in the mid- and high-frequency areas.