The invention provides a high-reliability unified power flow controller suitable for a double-circuit line. The high-reliability unified power flow controller comprises at least one parallel transformer, four converters, at least two series transformers, a parallel-side back brake loop, a series-side back brake loop and a direct-current public bus. At least one side of each series transformer is connected with at least one set of bypass switches in parallel. The alternating current side of at least one converter is connected with at least one series transformer through the series-side back brake loop to be connected into the line in series. The alternating current side of at least one converter is connected with the at least one parallel transformer through the parallel-side back brake loop to be connected into an alternating current system. The direct current sides of the converters are connected to the direct-current public bus through change-over switches. By means of the structure, multiple operation modes can be achieved, and when one converter breaks down, power flow of the double-circuit electric transmission line and the voltage of the parallel-side alternating current system can still be controlled; when two converters break down, control over the power flow of the double-circuit line can be achieved, and the good flexibility and the good reliability are achieved.