A UPS circuit, comprising a rectification phase leg (PL1) for rectification, which is used for converting a received alternating current into a direct current; inversion phase legs (PL21, PL22, PL23) for inversion, which are used for inverting the direct current output by the rectification phase leg into an alternating current; a failure detection device (D) which is used for detecting whether an inversion phase leg has failed; a redundant inversion phase leg (PL2a), with an input end thereof being connected to an output end of the rectification phase leg; and a control device which is used for receiving a signal sent by the failure detection device and is also used for enabling the redundant inversion phase leg to replace the inversion phase leg which has failed when a failure occurs.