Terahertz (Thz) photoconductive antenna is one of the most common devices for the generation of terahertz wave. Basing on the theory of current instantaneous impact model, the most effective way of improving the THZ wave power is to increase bias voltage and laser power, but the damage and the breakdown of photoconductive antenna is easy to occur when the bias voltage increases. In this paper, the physical breakdown mechanism of the antenna is researched on the different trigger positions and the different bias voltages. Trigger position is more likely to be breakdown because of the local high electric field stimulated by photon-generated carriers. Increasing bias voltage not only caused the current increasing quickly, but also ablated thermal breakdown of antenna electrode and resulted in antenna failure.