Abstract: With the increase of the access ratio of distributed generator（DG） in the distribution network, the problem of unplanned island caused by power outage is becoming more and more serious. Anti-islanding protection based on impedance measurement has gained increasing attention due to the advantages of low non-detection zone, less susceptibility to false activation, and less impact on power quality. However, variations in the number of DGs and mutual interference of islanding detection signals in multi-source distribution networks can lead to impedance measurements against protection failure Therefore, impedance angle measurement based on voltage synergistic injection is proposed to prevent islanding protection. Firstly, an islanding identification criterion based on impedance angle is designed to avoid the influence of DG running number change on protection. Then combined with the DG control strategy, the signal injection and impedance angle detection methods are designed. The proposed method makes each DG synchronously inject 0.04s islanding detection signal when detecting voltage changes, which avoids the mutual interference of the detection signals and reduces the impact on power quality. Finally, the simulation experiments based on RTDS show that the proposed method can accurately identify the power grid disturbance and island operation state, and effectively avoid the problems of the protection rejection and the false activation.