Unscheduled island state may bring some negative effects on system, user, and distributed generation itself. Existing island detection methods have the defect of low-reliability and low-sensitivity. This paper proposed a detection method based on signal correction by HV-side of distribution transformer which can solve the problem of the conventional phase shift detection error. A synchronized sine signal is produced and compared with the phase and frequency of distributed generation’s voltage signal, and the islanding detection criterion can be obtained.