A multimode Brownian oscillator model has been successfully applied for the first time to model the absorption and emission spectra of a conjugated polymer. Extremely good matches were obtained between the calculated and observed spectra for alkyl- and aryl-substituted indenofluorene-based polymers, revealing the underlying photophysics of the investigated polyindenofluorenes in solution as well as solid thin film. Structure variations, defects, and aggregation effects in polymer are shown to have major impacts on the changes of spectral features according to the MBO analysis. These results confirm that the MBO model is a valid method for analyzing the optical behavior of conjugated systems.