Based on a tight-binding approximation and transfer matrix method, we investigated the effects of the composition of nucleotide base pairs, on-site energies of the electrode and DNA-metal coupling strength on the current-voltage characteristic. The results indicate that the saturation current of DNA molecule which is composed of one single kind of nucleotide base pair is much higher than that composed of two kinds of nucleotide base pair. Meanwhile, the DNA molecule which is rich in G-C base pairs has higher electronic transport ability. When the bias is low, the on-site energies of the electrode have the effect of impeding charge injection. On the other hand, when the bias is high, the on-site energies of the electrode have the effect of enhancing charge injection. In addition, we can find that a stronger DNA-metal coupling does not always result in a larger saturation current. When tdm=td, there is a resonance injection, which is optimized for electron transport. When tdm departs from td, the resonance injection is reduced, which lead to the stronger of DNA-metal coupling at the range of tdm>td and the lower of saturation current of DNA.