As crucial equipment for multi-voltage DC interconnection， high-capacity high-voltage DC/DC converters can be used for voltage conversion and DC fault isolation. The DC/DC autotransformer capable of blocking DC faults and reducing their reliance on DC circuit breakers can significantly reduce construction costs. To this end， a DC/DC autotransformer topology with half-bridge modular multilevel converters （HB-MMCs） connected in series is proposed. The fault response after a DC bipolar short-circuit fault on each side of the DC/DC autotransformer is analyzed under two operating conditions： forward power feeding and reverse power feeding. A corresponding fault isolation strategy is proposed. Simulations are performed on the PSCAD/EMTDC platform for the bipolar short-circuit faults under different operating conditions. The simulation results show that the proposed autotransformer can block DC faults in two directions. In contrast to other types of DC/DC autotransformers， the proposed autotransformer is economical.