We have extended the photocurrent model on THz radiation generation from an external DC electric field biased and single-color femtosecond laser induced Ar-gas micro-plasma. The properties of THz radiation under different conditions have been analyzed. The simulation results show that the THz radiation from laser-induced plasma can be significantly enhanced by the introduction of an external DC electric field, which is linearly dependent on DC field. Furthermore, the THz polarization direction follows the direction of the external DC field. The electrons ionized and accelerated by both the laser field and the external DC electric field result in THz radiation, in which the external DC electric field has a dominant contribution. The theoretical and numerical investigations help us deeply understand the THz radiation generation process from DC electric field biased and femtosecond laser ionized gas plasma.