In microgrids, different entities will measure the voltages independently, which could cause that multiple PTs operate in parallel. In the island mode, the arc suppression coil or small resistant connected at neutral point in the main network that can clamp the voltage will be disconnected, and ferroresonance more likely occurs in this case. In order to improve the security of island mode of microgrid, this paper analyzes the mechanism of multi-PT ferroresonance and proposes suppression technology of ferroresonance. The analysis shows that ferroresonance cannot be reliably eliminated by using a series resistance in primary or secondary winding, however, can be reliably eliminated by using a ferroresonance-constraining resistance connected at neutral point. Based on the operation condition of microgrids, the selection and control of ferroresonance-constraining resistance are studied. The effectiveness of the scheme in the paper is verified by EMTP/ATP and practical operation. This work is supported by National Natural Science Foundation of China (No. 51777067).