This paper proposes a control strategy for seamless transition of the flexible interconnection device (FID) under distribution network faults. By adopting grid-forming control in both grid-connected and stand-alone modes, the FID provides the stable and seamless voltages for the critical loads during the mode transition. In addition, all the converters of the FID have the ability to regulate the DC voltage, which can ensure the spontaneous stabilization of DC voltage after a fault occurs. Then, by designing a coordination strategy for the FID control system, the power flow of the FID is regulated and the DC voltage is guaranteed to be within an acceptable range. Compared with the traditional control strategy, the FID does not require the control switching or new DC-voltage-regulation converter selecting under a distribution network fault, which makes the mode-transition operation simplified and seamless. Finally, the effectiveness of the proposed strategy is verified by simulation.