Stability and dynamic behavior of negative differential conductivity in thyristors are studied in this paper, which aims to clarify the mechanism of chaotic phenomena in the thyristor. Firstly, a spatio-temporal model of the thyristor is established, and the boundary condition of the system is obtained based on the linear stability analysis. The results show that the instability of thyristor is not only determined by the characteristics of negative differential conductivity, but also depends on the external conditions. Computer simulation is made to verify the proposed view for different external control parameters. The theoretical results are also confirmed by experimental measurements. So, the mechanism of chaotic phenomena in thyristor is clearly explained.