Abstract: As a typical multiport converter, triple active-bridge (TAB) DC-DC converter is widely used in distributed DC system. The open-circuit fault (OCF) of TAB seriously threatens the system safety operation. This article proposed an OCF diagnosis and tolerant method for TAB converter to reduce the impact of OCF. Firstly, normal mode and OCF mode of TAB are analyzed. Base on the analysis, the average value of the midpoint voltage of each bridge arm can be used as an OCF diagnosis indicator. The DC bias of transformer winding current are eliminated by blocking the gate-driving signal of the faulty bridge arm. Then the blocking mode is analyzed. Finally, fault-tolerant operation is achieved by regulating the phase shift angle range. If the OCF occurs on the DC bus side, the phase shift angle is adjusted to ensure that the DC bus still absorbs power. If the OCF occurs on the energy storage side, the phase shift angle is adjusted to ensure that the energy storage cut off. The OCF can be diagnosed accurately and quickly according to the experimental results. The impact of OCF can be reduced, which promotes the reliability of TAB converters.