SF8TBT and LF8TBT synthesized from analogous precursors have different photovoltaic performances. On account of rationalizing the experimental properties, density functional theory and time-dependent density functional theory are employed to investigate the electronic and structural properties of the molecules. The researches involve in geometry properties, absorption spectra, frontier molecular orbitals, hole forming and transporting properties, and exciton dissociation abilities at the molecule/PC71BM interface. The consequences indicate that the better photovoltaic performances of SF8TBT primarily originate from stronger hole forming ability, better hole transport properties, and more favorable exciton dissociation ability. In conclusion, the theoretical results coincide well with the experimental ones.