There is a growing interest in developing low-band gap conjugated polymers containing alternating units of π-electron-donating/accepting capabilities due to their wide applications in electronic devices. Energy levels and band gap of the organic materials are the dominant factors to the performance of these organic electronic devices. In this study, three strategies including (1) variation of donor/acceptor contrast, (2) modification of donor/acceptor compositions and (3) modification of coplanarity on donor/acceptor conjugated copolymers are proposed for band engineering of donor/acceptor conjugated copolymers. Electronic and optical properties of conjugated copolymers are investigated by using density function theory (DFT) and semi-empirical ZINDO calculations. A remarkable linear correlation is found between the amount of charge transfer between the donor-acceptor pair, the band gap, the bandwidth, and the oscillator strength of S0->S1 electronic transition (ground state to first excited state) of the copolymers. Strong π-electron withdrawing substituents on the acceptor moiety effectively reduce the band gap of the copolymers. However, the reduction of band gap is frequently accompanied by a linear reduction in bandwidths and in the oscillator strength of S0->S1 transition. Fine tuning of the electronic properties could be achieved by varying the composition of π-donor/acceptor on D/A conjugated polymers. In the case of strong donor/acceptor contrast results in non-ideal behaviors of electronic properties. But remarkable linear correlations of electronic properties against different D/A compositions were observed when the block size of one parent monomer in strong D/A contrast system is fixed. The positions of alkyl side chains on conjugated polymers may change the co-planarity of the copolymers and thus significantly affect the band gap and energy levels. In addition, HOMO and LUMO levels could be individually controlled in strong donor/acceptor systems by adjusting co-planarity of the copolymer. Our results suggested that three strategies proposed in this study would be effective molecular design rules toward desired electronic and optical properties for low band gap conjugated polymers.