To prevent the series transformer and inverter valve from over-voltage in the Unified Power Flow Controller (UPFC) during system fault, the two wingdings of series transformer should be paralleled with fast bypass devices such as mechanical bypass switch, thyristor bypass switch (TBS), MOV and etc. TBS that utilizes the fast conduction of thyristor is studied. The overvoltage protection principle of UPFC is introduced first, then taking the Nanjing UPFC project as an example, the structure and parameters of TBS are designed, the components selection of TBS is introduced and the overvoltage matching between the BOD protection of TBS and MOV is checked. The performance of TBS is verified by maximum short-time current withstand test in the synthetic test circuit and temperature rise test. In the end, the structure and detection process of the state detection system of the TBS are introduced.