The third harmonic generation spectrum (THG) of trans-polyacetylene exhibits two peaks at 0.6eV and 0.9 eV respectively. None of the existing theories can give a quantitatively satis-factory interpretation. By considering properly the excitation-dependent damping, we can get the second peak at 0.9eV. The damping of excited states that we depict in this paper due to electron-phonon interaction of polymer varies with the excited states. According to this dam-ping, the theoretical results of THG agree with the observed spectrum very well.