In this paper, on the basis of polydiketopyrrolopyrrole-bithiophene (PDPP-2T, P1), three groups of pyridine (Py), thiazole (Tz), and furan (THF) were selected to construct a D-π-A copolymer P2, P3, P4, and the photoelectric properties of the four polymers were calculated by the first-principle of density functional (DFT). The calculation results show that the band curvature becomes larger after the introduction of the π bridge, which is helpful to improve the conductivity of the copolymer, but the band gap increases. The short-wave absorption peaks of the light absorption peak are red-shift, and the long-wave peaks of P2 and P4 are blue shift, while the long-wave peak of P3 does not change much. And the intensity of the long-wave absorption peak is weakened. The influence of three kinds of π bridges on the charge transfer between D and A units was studied by Bader charge analysis. It was found that the charge transfers of P2 and P3 decreases and P4 increases slightly after the introduction of π bridges. It is an gain-electron unit or a loss-electron unit, which shows that the introduction of a π bridge has a certain effect on the charge transport of the D/A copolymer. In short, it is a problem that needs to be considered comprehensively to increase the photoelectric properties of the copolymer by introducing a π bridge.