The offshore DC microgrid system has requirements of high reliability, low power capacity, low line impedance, and high fault-current rising rate for its detection. To address these issues, a kind of combined short-circuit-fault detection method is proposed, which consists of the differential current method and the directional current method. Short-circuit fault of bus and branch is detected by using the basic law of current, current direction characteristics and variation magnitude of each branch fault. The specific implementation method is given. A scaled-down experiment platform is established. By setting short-circuit fault through controlling IGBT ( insulated gate bipolar transistor), experiments on short-circuit fault of bus and branch are implemented. Experimental results show that the removal time of bus short-circuit fault is 0. 09 ms, and the removal time of branch short-circuit fault is 0. 22 ms. The detection speed is faster than that of the current DC ring micro-grid system short-circuit fault, and it also has good selectivity and wide coverage.