The invention provides a convertible static synchronous series compensator. As to N circuits, the convertible static synchronous series compensator comprises at least one parallel transformer, a voltage source converter, N series transformers and at least (N+1) changer-over switches, wherein at least one side of each series transformer is in parallel connection with at least one bypass switch, an alternating current side of the voltage source converter is connected with the at least one parallel transformer through at least one change-over switch and then parellelly connected with an alternating current system, meanwhile, the alternating current side of the voltage source converter is connected with the N series transformers through at least N change-over switches, and the N series transformers are serially connected with the N circuits respectively. According to the convertible static synchronous series compensator, by means of only one voltage source converter, alternating current voltage control of a power grid and power flow control of different circuits can be achieved, and good economy and flexibility are achieved.