The ability of a thyristor to withstand overvoltage is weak, and transient overvoltage can cause the breakdown of the thyristor. The thyristor is one of the core devices of DC transmission. First, this paper analyzes the influence of thyristor overvoltage breakdown on the DC transmission system and the necessity of configuring thyristor overvoltage protection on the DC converter valve. Second, BTC and BOD thyristor overvoltage protection methods are calculated and analyzed theoretically, and a simulation model is built by PSPICE software for analysis. Finally, an experimental circuit is built in the laboratory to test the performance of two overvoltage protection circuits. Studies show that both protection methods can achieve the requirements of thyristor overvoltage protection. This work is supported by Science and Technology Project of China Southern Power Grid Company (No. CGYKJXM 20160082).