We theoretically investigated the physical properties, including the frontier orbital and excitation energies, for thiophene-based semiconducting polymers composed of donor and acceptor units. Orbital analysis revealed that remarkably different behaviors of frontier orbital energies with respect to the degree of polymerization stems from the distribution of the frontier orbitals, which is insightful information for controlling the ionization potentials and electron affinities of semiconducting polymers. We also successfully estimated the frontier orbital energies of the polymers through a simple Hückel theory-based analytical model parametrized from calculations of relatively small oligomers. This simple model allows us to predict the highest occupied molecular orbital-lowest unoccupied molecular orbital gaps of a polymer at a low computational cost. The simulated absorption spectra of the thiophene-based semiconducting polymers were compared with the experimental spectra. The theoretically designed polymers were also investigated in terms of their frontier orbital energies and absorption spectra toward synthesizing promising polymers.