Varying composition of pi-donor/acceptor moieties has been considered as an effective strategy for fine-tuning of the electronic properties of D-A conjugated copolymers. In this study, the change of optoelectronic properties with the change of donor/acceptor ratios is investigated on the basis of first-principles density functional calculations. Copolymers containing moieties of similar g-electron donating and/or accepting capabilities, e.g., thiophene (T)-methoxythiophene (OT), exhibit a linear dependence of electronic properties (especially, HOMO/LUMO, band gap, and bandwidth) on the D/A content. In contrast, for strong D/A contrast systems, e.g., thiophene (T)-thienopyrazine (TP), the electronic properties vary nonlinearly with D/A compositions. However, when the block size of one parent monomer in a strong D/A contrast system is fixed, the variation of electronic properties shows a remarkable linear correlation against D/A compositions. We found that the deviation of electronic properties from a linear composition dependence is dominated by the strength of orbital interactions between D and A. Weak orbital interactions between D and A moieties tend to lead to a nonlinear composition dependence. Our results provide useful insights for band gap tuning through the adjustment of D/A compositions in D-A conjugated copolymers.