The reverse recovery characteristics of the high power thyristor becomes increasing important to the converter valve designers as commutation failure often occurs in its reverse recovery process. Therefore, the recovery protection of converter valve is provided by thyristor control unit. The design of thyristor control unit is associated with the relationship between reverse recovery characteristics and external operating conditions. In this paper, the thyristor reverse recovery characteristics has been investigated to determine the reverse recovery time based on analyzing and testing the reverse recovery process of thyristor. Various types of recovery as well as some important circuit influences on the recovery characteristics of high power thyristors are considered. The effects of peak current during conduction, conduction pulse width, and snubber circuit are presented. Thereafter, the influences of thyristor reverse recovery time on the design of thyristor control unit have been investigated.