D-A type copolymer as an organic polymer solar cell electronic material in recent years has attracted wide attention. In order to improve the efficiency of energy conversion, many active layer materials, especially the donor materials, have been designed and synthesized. By inducing the different donor and acceptor units, the absorption spectrum can better match with the solar spectrum and the carrier mobility can increase. In this paper, by using the density functional theory method, we investigate the electronic structures and optical absorption spectra of D-A and D--A copolymers. Benzodithiophene (BDT) as the electron donor unit, and dibenzothiophene (BT) as the electron acceptor unit are used to simulate D-A (PBDT-BX, X = O, S, Se, Te) copolymer systems; and D--A (PBDT-DTBX, X = O, S, Se, Te) structures are constructed with thiophene ring as a bridge between D and A. Firstly, our calculation results indicate that when X is replaced separately by elements O, S, Se and Te in D-A copolymers, the LUMO levels move close to the Fermi level, while the changes of the HOMO energy levels are relatively small, resulting in the band gap decreasing gradually. Then, the analysis of the density of states (DOS) shows that the contribution of LUMO comes from the BT unit and HOMO from the BDT unit. Also the difference in charge density shows that the thiophene ring enhances the charge transfer between BT and BDT. Besides, the studies of the optical absorption spectrum reveal that there appear two strong absorption peaks with the increase of atomic number of X, of which one is at about 4.0 eV and has no obvious change, and the other increases intensively and is red-shifted. Moreover, compared with the D-A structure, the D--A structure has the band gap that will decrease obviously and has a lowest value when X is Te. The optical absorption peak also increases significantly as the atomic number of oxygen group elements increases and peak position is red-shifted. The range of optical absorption peak is mainly from 703.9 to 519.4 nm. According to the absorption spectrum and DOS the optical absorption peak at about 4.0 eV is mainly contributed by the BDT unit. Overall, our findings provide a good understanding of mechanism about the red-shift of optical absorption spectra of copolymers and can serve as guidance for organic polymer design in photovoltaic cell experimentally.