Active constrained layer damping treatments were applied to the secondary mirror supporting structure for space telescopes in order to improve its dynamic performance. Active constrained layer damping treatments, which apply piezoelectric actuators and viscoelastic materials to flexible structures, promise to be an effective means of vibration suppression in flexible structures. A finite element model for active constrained layer damping treatments was developed and a proportional derivative control law was applied on piezoelectric ceramics. Effects of key parameters, such as viscoelastic layer thickness and control gain were investigated. It is shown that the increase of viscoelastic layer thickness and control gain could both enhance the damping capability. However, the actuation ability of piezoelectric ceramics is reduced by the viscoelastic layer. Analysis results of the telescope indicate that natural frequencies of the supporting structure decrease slightly after employing active constrained layer damping treatments. However, the frequency response at the secondary mirror is suppressed apparently and the surface accuracy is improved as well.