Absorption and fluorescence spectra of the poly(propyleneoxide)–poly(phenylene ethynylene) with long polymer chains in the experiment are investigated by the Franck–Condon simulation with small polymer chains in the theory. Electronic calculations with time-dependent density functional (TDDFT) indicate that vibronic spectra come from S1(ππ*) excitation and de-excitation. Good agreement between Franck–Condon simulated and experimentally measured absorption and fluorescence spectra in three kinds of solutions is achieved. However, vibronic spectral analysis reveals that vibronic motion mechanisms are quite different for simulated absorption and fluorescence spectra. For absorption spectra, vibronic spectra are dominated by one most active mode localized in triple bond carbon–carbon stretch motion on backbone of the poly (phenylene ethynylene), and simulated absorption spectra indicate that the long chains can be as stable as short chains on the electronic ground state in comparison with experimental spectra in all solvents. In the case of fluorescence spectra, this most active mode is spitting into several equally active modes with very close values in vibrational frequencies, and thus, the spitting modes and mode couplings among those spitting dominate vibronic spectra. Simulated fluorescence spectra indicate that the only short chains are stable on the electronic excited state in comparison with experimental spectra in all solvents. Nevertheless, the present simulation concludes that both absorption and fluorescence spectra are induced mostly by the active normal mode in triple bond carbon–carbon stretch motion on backbone of poly(phenylene ethynylene). The present work provides physical insight for analyzing observed vibronic spectra of similar conjugated polymers and for designing novel optoelectronic materials.