Dielectric elastomers (DEs) are one of the most potential electroactive polymer (EAP) used as highperformance actuators. A DE Subjected to a voltage can generate large thickness and area expansion with a maximum area strain of 1600%. As DE has natural damping properties and its deformation is then dependent to time, its dynamic performance, energy conversion and large deformation are definitely influenced by damping. A free energy model is developed to study the dynamic characteristics of a dielectric elastomer actuator undergoing inplane deformation subjected to the combined loadings of a mechanical press and an electric eld. The numerical results including the oscillation, phase diagrams and Poincaré maps are presented to show the inuence of the damping on the nonlinear dynamic characteristics of the dielectric elastomer. The numerical results indicate that the damping effect could cause the dynamic responses to constant vibration and decrease the amplitude. The phase paths are all presented in closed regions and the points of Poincaré map are finite, indicating that the DE system experiences a nonlinear periodic oscillation, and the dynamic oscillation of the DE system is stable. These conclusions provide the basis for the exploration of highperformance dielectric elastomers under dynamic mechanical and electrical loads.