We theoretically investigate the optoelectronic properties of graphene driven by an external DC electric field caused via in-plane source-drain voltage. It is found that the Drude part dominates the optical conductivity in the low-frequency region, whereas the plasmon effect can give rise to a relatively weak but observable the optical conductivity in the high-frequency region. Both the Drude and plasmon-induced parts of optical conductivity can be observed in the terahertz (THz) bandwidth. Drude part decreases monotonously with increasing of the photon frequency, the plasmon-induced part exhibits a peak at about ω~1 THz due to resonant plasmon-photon interaction in graphene. Moreover, we examined the dependence of plasmon-induced optical conductivity on the driving electric field and electron density and find that it depends sensitively on these parameters. It is indicated that the optical conductivity resulted from Drude and plasmon-induced parts in graphene can be tuned efficiently not only by electric gating but also by source-drain voltage.