Abstract: A new composite energy dissipator made of two O-shaped steel plate metal dampers and one high damping viscoelastic damper was developed. The construction details and working mechanisms were introduced. Cyclic loading tests on the dissipator were carried out. The test results show that the composite energy dissipator possesses a large deformation capability and a full hysteresis curve. The mechanical properties of the dissipator are stable and insensitive to loading frequencies. It has the advantages of the velocity-dependent damper and displacement-dependent damper. When the deformation is small, the viscoelastic damper plays the major role in energy dissipation, and the O-shaped steel plate metal dampers contribute to stiffness. When the deformation is large, two types of dampers dissipate the seismic energy together. Compared with the dampers of a single type, the composite energy dissipator offers a much larger damping force and seismic safety margin. The calculation model for the composite dissipator was established by using the Bouc-Wen model. The calculation results were in good agreement with experimental results.