To implement a highly flexible polymer solar cell (PSC), the mechanical properties of the material constituting the active layer are considered crucial, having a significant influence on the stability of the device. In this study, the mechanical properties of a main-chain conjugated copolymer (PBDT2T-b-PNDI2T) containing donor- and acceptor-based macromolecular species in the polymer backbone was measured and compared with that of donor and acceptor polymer blend film (P(BDT2T):P(NDI2T)). It was observed that the PBDT2T-b-PNDI2T active layer exhibited a higher crack-onset-strain and toughness compared to P(BDT2T):P(NDI2T) active layer. Correspondingly, the YTRON/polyethylene naphthalate (PEN) based flexible PSC with a PBDT2T-b-PNDI2T active layer was used, which exhibited superior mechanical stability under multiple bending conditions. The results of this study indicate that flexible PSCs with excellent durability can be developed by replacing the polymer blend used for all-PSCs with PBDT2T-b-PNDI2T as the active layer material for highly efficient future applications.