Recently, all-polymer solar cells (all-PSCs) have realized significantly improved power conversion efficiencies (PCEs) through the development of various high-performance polymer donors and polymer acceptors. In addition, their device efficiency has been maximized using numerous strategies, such as thermal annealing and adding additives. In this study, we synthesized a new low-bandgap polymer acceptor (PTPB-TT) for application in all-PSCs. PTPB-TT exhibited adequate energy levels as a polymer acceptor and efficient complementary absorption behavior in the blend film. Moreover, we introduced a ternary system with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the third-component acceptor to improve the light-harvesting properties and morphology of the binary blend film. PC61BM realizes the cascade energy levels in the ternary system, enabling more efficient charge transfer between the donor and the acceptor polymers than in the binary system. Therefore, the corresponding ternary devices displayed a relatively high PCE of 8.03% compared to the binary all-PSCs.