The present invention discloses an auxiliary power supply with a bidirectional power supply, comprising a bidirectional isolating converter, a first non-isolating converter, a second non-isolating converter, a first combining selector and a second combining selector, wherein the first combining selector is respectively connected with a primary side pressure-wire, the bidirectional isolating converter and the first non-isolating converter; and the second combining selector is respectively connected with a secondary side pressure-wire, the bidirectional isolating converter and the second non-isolating converter. According to the embodiment of the present invention, when power supply of the primary side and the secondary side is abnormal, the auxiliary power supply still can supply power fora primary side auxiliary power supply and a secondary side auxiliary power supply normally with the only one bidirectional isolating converter, thereby reducing cost and size for the isolating, and raising efficiency of auxiliary power converting during normal power supply of the primary side and the secondary side.