Based on the method to constitute a PN junction with the addition of external positive and negative voltages at both ends of graphene sheets, the relationships among the optical conductivity and optical absorption with the external voltage, disorder broadening width, and temperature are studied by adjusting the gate and bias voltages. The study results show that the optical conductivity of graphene has negative value in the terahertz region and the light transmissivity increases. In the terahertz region, the optical conductivity depends on the external bias voltage and the disorder broadening width monotonously, but on the temperature complicatedly. Under a given applied voltage, the optical transmissivity can be enhanced if an appropriate temperature is chosen. The study results provide theoretical references for the applications of graphene in the terahertz region.