In this study, we apply Koopmans’ theorem based optimally tuned (OT) long-range corrected density functional theory (LC-DFT) combined with a polarizable continuum model (PCM) to describe the electronic and optical properties of the non-fullerene acceptor, Y6. When optimizing the range separation parameter μ, we found that structural changes within the same molecule have only a small effect on μ value, while the siginificant difference in the μ value is observed between gas-phase and PCM environments, with smaller μ values under PCM conditions. The μ value optimized using PCM shows good agreement with experimental ionization potential (IP), electron affinity (EA), and fundamental gap of solid-state Y6. In addition, time-dependent (TD)-OT-LC-ωPBE calculations accurately reproduce the near-infrared absorption maxima and the red-shift observed when changing from solution to thin-film conditions. These results indicate that OT-LC-DFT is a practical and reliable approach for predicting electronic structure and optical properties in condensed-phase environments.