Terahertz (THz) radition has attracted significant attention toward applications on such as biology, imaging, sensing and communications due to its unique characteristics. Femtosecond laser filamentation is one of the important approaches to obtain THz pulse with ultra-broad bandwidth and high electric field. With the rapid development of femtosecond laser technology, plasma based THz generation by femtosecond laser has been well studied. Two theoretical models have reported for understanding the physical mechanism of THz pulse generation, namely the photocurrent model and the four wave mixing model, respectively. By applying an external static electric field to the laser-induced plasmas filament, the enhancement of THz radiation has been reported by several orders of magnitude. However, the physical mechanism of laser-induced THz emission under the external static electric field remains unclear because of the complex interaction of laser plasmas. In this paper, based on the photocurrent model, we analyze and numerically simulate the properties of THz emissiom from a DC-biased single-color femtosecond laser ionized gas micro-plasma. The results show that the photocurrent model is effective in interpreting the THz generation of DC-biased laser-induced gas plasmas.