The invention belongs to the field of polymer photoelectric materials and discloses a block copolymer with naphthalene bisimide and indacen cyanindanone as a main chain structure and application of the block copolymer in an organic photovoltaic device. A structural formula of the copolymer is shown in the description, wherein n is a positive integer smaller than or equal to a million, X is largerthan 0 and smaller than or equal to 1, R1, R2, R3, R4, R5, R6 are same or different alkyl chain, A and B are of a conjugated unit structure and are identically or differently at least one of thiophene, furan, bithiophene, bifuran, thienofuran, thiophthene, o-furan and structural derivatives thereof, and C is an aromatic ring and is at least one of benzene, thiophene, furan, selenophen, pyrrole, thiazole, imidazole, pyridine, pyrazine, pyrimidine and pyridazine. The polymer can have relatively regular stacking and also relatively high absorption coefficient, can be used for an efficient organicphotovoltaic device as an efficient electron acceptor and effectively improves the performance of the device.