By placing a pair of linear electrodes at the focal point of a femtosecond laser, the plasma generated through the laser-ionized air is placed in the bias direct-current (DC) field generated by the linear electrode. We observe the terahertz characteristics of plasma in the bias-electric field in this case. By scanning the linear electrodes and observing the terahertz wave intensity, we obtain the optimal electrode position for enhancing the amplitude of terahertz wave. Results suggest that as the laser power increases, the optimal position moves in the direction of light propagation. This work not only studies the characteristics of terahertz signal generated through plasma under the modulation of linear electrodes, but also reveals the physical characteristics of plasma.