The invention discloses an open-delta injection type impedance detection island discriminating method, which comprises the steps of: A, using a small-capacity star-delta transformer at a grid-connected point of microgrids, and injecting a measurement signal at a triangle side; B, sampling a three-phase voltage reflected by the triangle side of the star-delta transformer and a zero-sequence ring current in the triangle; C, calculating zero-sequence impedance of a system at an injection point in real time, and comparing the zero-sequence impedance with zero-sequence impedance of the system at the injection point before a delta T moment; D, and performing island discrimination by adopting a method of comparing three-phase phase-splitting zero-sequence impedance variations before and after island formation. The open-delta injection type impedance detection island discriminating method has the beneficial effects that: 1, the circuit is simple and convenient in implementation, an additionalprotection circuit is provided, and elements are not prone to damage; 2, the island discrimination can be performed at the designated grid-connected point of the microgrids without depending on an inverter; 3, the judgment is accurate and fast, the protection dead zone is small, the possibility of refusal operation and misoperation is small; 4, and the influence on electric energy quality of the power grids is small.