A unified power flow controller "UPFC" suitable for a double-circuit transmission line comprising a first transmission line and a second transmission line, the UPFC comprising: at least one parallel-connection transformer, three current converters, at least two series-connection transformers comprising first and second sides including a first series-connection transformer with its first side connected to the first transmission line and a second series-connection transformer with its first side connected to the second transmission line, at least one group of by-pass switches (9,10) and a direct current common bus; wherein at least one side of the series-connection transformers is connected with the at least one group of by-pass switches in parallel; characterized in that: the UPFC further comprises: a series-connection side switching circuit configured to connect two of the three current converters with the at least two series-connection transformers so that the alternating current side of one of the two current converters be connected to the second side of the first series-connected transformer and the alternating current side of the other converter of the two converters be connected to the second side of the second series-connected transformer; a parallel-connection side-switching circuit configured to connect the alternating current side of the three converters to an alternating current system through the at least one parallel-connection transformer; and a transfer switch configured to connect the direct current sides of the three current converters to the direct current common bus. at least one parallel-connection transformer, three current converters, at least two series-connection transformers comprising first and second sides including a first series-connection transformer with its first side connected to the first transmission line and a second series-connection transformer with its first side connected to the second transmission line, at least one group of by-pass switches (9,10) and a direct current common bus; wherein at least one side of the series-connection transformers is connected with the at least one group of by-pass switches in parallel; characterized in that: the UPFC further comprises: a series-connection side switching circuit configured to connect two of the three current converters with the at least two series-connection transformers so that the alternating current side of one of the two current converters be connected to the second side of the first series-connected transformer and the alternating current side of the other converter of the two converters be connected to the second side of the second series-connected transformer; a parallel-connection side-switching circuit configured to connect the alternating current side of the three converters to an alternating current system through the at least one parallel-connection transformer; and a transfer switch configured to connect the direct current sides of the three current converters to the direct current common bus. a series-connection side switching circuit configured to connect two of the three current converters with the at least two series-connection transformers so that the alternating current side of one of the two current converters be connected to the second side of the first series-connected transformer and the alternating current side of the other converter of the two converters be connected to the second side of the second series-connected transformer; a parallel-connection side-switching circuit configured to connect the alternating current side of the three converters to an alternating current system through the at least one parallel-connection transformer; and a transfer switch configured to connect the direct current sides of the three current converters to the direct current common bus.