With the development of HVDC transmission technology, HVDC converter valves are widely used in many areas including the connection between systems with different frequencies. Thyristor is the core component in HVDC converter valve and its reliability will influence the operation of HVDC converter valve. However, the structure of thyristor is fragile under over-voltage condition. On the one hand, external shock such as HEMP will cause over-voltage condition inside the converter valve. The changing rate of voltage is so large that structure of thyristor may be destroyed. On the other hand, when the thyristor is in reverse characteristic recovery phase, a large reverse recovery voltage overshoot occurs. This over-voltage condition will cause damage to thyristor because thyristor is fragile in this phase. Based on these analysis above, in order to reduce the damages caused by over-voltage and design some methods to realize the online- monitoring, this paper summarizes the fault characteristics of thyristor under over-voltage condition.