Based on the modified mixed Hellinger-Reissner(H-R) variational principle for elastic bodies with damping, the state-vector equation with parameters is directionally derived from the principle. A new solution for the harmonic vibration of simply supported rectangular laminates with damping is proposed by using the precise integration method and Muller method. The general solutions for the free vibration of underdamping, critical damp and overdamping of composite laminates are given simply in terms of the linear damping vibration theory. The effect of viscous damping force on the vibration of composite laminates is investigated through numerical examples. The state-vector equation theory and its application areas are extended.