We report that the intensity of the terahertz pulse is significantly enhanced by placing the femtosecond laser induced air plasma into a high-voltage DC electric field, which is generated by two parallel metal electrode plates. We experimentally demonstrate that the intensity of the terahertz pulse enhances linearly with the increase of the external electric field magnitude. We also find that the width of the effective spectrum decreases as the bias voltage increases, and the center frequency shifts to the low frequency. Furthermore, we find an optimal solution for applying the external electric field by changing the angle of the two parallel metal electrode plates. We speculate that the above phenomenon is mainly due to the plasma current generating terahertz and the bias electric field can drive the current in the plasma to enhance the terahertz signal.