To acquire the characteristic parameters which demonstrate the state of high voltage thyristor for clarifying its failure mechanism, an experiment platform which can not only generate high voltage pulses to thyristor at different moments during reverse recovery period, but also accurately measure its key characteristics is established. The change of characteristic parameters during the pulse impact process is studied, and the disassembly of the degraded and failed thyristors is analyzed. The results show that the damage of the thyristor can be mainly manifested as the reduction or loss of its blocking ability undertake voltage pulses in the middle of the reverse recovery period. Leakage current can be used as a characteristic parameter to estimate the state of the thyristor. For thyristors undertake voltage pulses in the beginning and middle of the reverse recovery period, obvious breakdown spot can be seen on the silicon wafer surface. Obvious breakdown points can be seen on the device chips that have been impacted and failed in the early and middle reverse recovery period. Circular spots formed by thermal stress can be seen on the device chips that are impacted in the middle of the reverse recovery period and have degraded blocking ability. Reverse recovery avalanche breakdown flash marks can be seen at the junction between the edge of the device chip and the insulating rubber that have been impacted and failed at the end of the period.