Abstract: In order to study the restoring force model of seismic-damaged double-deck viaduct frame piers strengthened with Carbon Fiber Reinforced Polymer (CFRP) and Externally Wrapped Steel Section (EWSS), the destructive tests on one prototype reference specimen and one strengthened specimen without pre-damage and two strengthened specimens with different degrees of pre-damage were carried out under low-cyclic loading. Hysteresis curves of specimens were obtained and skeleton curves of specimens were extracted, and the hysteresis characteristics were analyzed. The skeleton curve models considering the initial damage were proposed, in which the elastic segment and hardening segment are double-fold lines, and the descending segment is an exponential function line. The regression fitting method of the test data was used to quantitatively describe the degradation law of the unloading stiffness of the hysteresis curve of the specimens. Considering the strength degradation under the same level of loading and the characteristics of fixed-point orientation, the restoring force models were established. The study results show that the hysteresis curves of specimens strengthened with CFRP and EWSS are obviously pinched with the inverted S-shape. Each hysteresis loop intersects at both the positive point and the negative point of the skeleton curve, and the load at the point is 0.25 times the yield load. After the peak load, Bauschinger effect of EWSS occurs. For strengthened specimens with initially moderate and severe seismic-damage, the parameter k of exponential function of descending segment of the proposed skeleton curve model is recommended to be 3.6 and 3.4, respectively. The calculated results of the proposed skeleton curve model and the established restoring force model are in good agreement with the experimental results, which can provide a basis for the elastoplastic seismic response analysis of similar structures.