All polymer photovoltaic cells offer unique potentials owing to the chemical and electronic tunability of both polymer donors and polymer acceptors. Compared with the numerous a-conjugated polymer donors, choices of pi-conjugated polymer acceptors are limited for photovoltaic applications. Although 2D-conjugated benzo[1,2-b:4,5-b']dithiophene (BDT) units are widely used as building blocks in highly efficient donor polymers in recent years, polymer acceptors based on these units have not been reported yet. Herein, a novel 2D-conjugated polymer acceptor (PBDTNDI-T) based on naphthalene diimide (NDI) and alkylthiothiophene-substituted BDT was designed, synthesized, and in-depth characterized. The polymers' photophysical, electrical, crystallinity, and morphological properties are addressed in homopolymer and blend films and well correlated with device performance. Under the weight ratio of 1.5:1 and 3 vol % of 1-chloronaphthalene, the PBDTNDI-T-based all polymer photovoltaic device exhibited a desirable PCE of nearly 3%, which is ascribed to the relatively high domain purity and small domain characteristic length observed by resonant soft X-ray scattering (R-SoXS) characterizations. These results demonstrated PBDTNDI-T is a novel polymer acceptor and also promising candidate material for efficient energy-related applications.