Ternary blends composed of two donor absorbers and a complementary absorbing material provide an opportunity to enhance the short-circuit current and thus improved the power conversion efficiency (PCE) of polymer solar cells (PSCs). The used of PBT-T-DPP as the complementary electron donor in poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyricacid methyl ester (PC61BM) blend to construct the ternary PSCs. The PCE of the optimized ternary blend PSCs (with 5 wt% PBT-T-DPP) reached 3.65%, which was 15.51% higher than that of the PSCs with binary blends of P3HT:PC61BM. The ternary blend with 5 wt% PBT-T-DPP exhibited well-developed morphology while the blend with 15 wt% PBT-T-DPP showed phase separation with large-sized domains. Improved photovoltaic performance of ternary PSCs was mainly due to complementary absorption of two donors as well as facilitating charge separation and transport while suppressing charge recombination through a combination of cascade energy levels and optimized device morphology.