This paper analyzes the active vibration control of sandwich beams using Active Constrained Layer Damping (ACLD). The finite element model of the viscoelastic sandwich beam combines finite element method with the Golla Hughes McTavish (GHM) model, using a 2-node 8 degrees freedom element. The finite element model is validated by the first four natural frequencies of the model in the literature, and the governing equations of sandwich beams are generated based on the Hamiltonian principle. The physical space dynamic condensation technique and state space complex mode decoupling method are employed to reduce the order of the structural model. This is necessary because free degree of the finite element model is too high to directly control the structure’s vibration. It shows that the fundamental physical characteristics of the structure may remain largely unchanged while the physical and state spaces are jointly reduced. We investigated how the positions and coverages of ACLD patches impact on the active control, vibration damping of viscoelastic sandwich beams.