The failure modes of thyristors are various, which are usually caused by thermo-mechanical fatigue, package aging, chip defect expansion, environmental stress corrosion and other factors. These failure modes not only affect the performance of the device, but also may cause system-level failure. Therefore, it is of great significance to study the failure mechanism and take targeted protective measures to improve the reliability and service life of thyristors. In this paper, the short-circuit current experiment method is used to simulate the operation of the thyristor when the abnormal large current passes through the device when the short-circuit fault of the converter valve bridge arm or the short-circuit fault of the load occurs. The results show that there is no abnormality in the appearance of the silicon chip in the 5 min group under the action of 500A current, and the control circuit can be normal after the electrical test. In the 15 min and 25 min groups, the Al coating in the central area of the silicon chip surface melted. After electrical testing, the device performance decreased significantly, and the circuit could not be completely turned off. In the 30 min group, except for the melting of the aluminum sheet, the silicon chip and the molybdenum sheet were bonded, and the device completely failed. After microstructure observation, it was found that the interface between molybdenum sheet and silicon sheet was no longer clear, and MoSi2 was formed by reaction.