The thyristor bypass switch ( TBS ) is an important protective device in the UPFC system. It can realize the fast bypass and isolation protection of the UPFC in the power system failure. Because TBS is put into operation only when the system fails, it is always in hot standby when the system is running normally. Because of the working characteristics of the TBS, it can not realize the state of the thyristor circuit element by on-line detection means as other devices. In this paper, the commonly used thyristor damping loop parameters measurement methods are compared, combined with the characteristics of TBS thyristor stage damping loop, a method for measuring the RC resistance capacitance of the TBS thyristor circuit based on the AD5933 impedance measurement chip is proposed, which has the advantages of fast, accurate, reliable and so on.