Considering the isolation strategy of microgrid and utility power, the paper proposes a new protection scheme for external short-circuit fault of microgrid based on positive-sequence fault component principle. The principle extracts the positive-sequence fault components of two terminals of tieline between microgrid and utility power and calculates their impedances, then uses the impedance angle as new fault criterion to identify the external fault of microgrid. Without considering the load current and fault resistance, the principle overcomes the bidirectional characteristic and finiteness of short-circuit current. Meanwhile, the current magnitude of positive-sequence fault components is used as trigger criterion, thus the comprehensive protection scheme is presented. A microgrid model connected to utility power is built by PSCAD/EMTDC, and all kinds of fault type are simulated, the simulation result shows that the protection scheme is correct and effective.