Abstract: In order to address the problems encountered by the protection in active distribution network, particularly the absence of the selectivity and reliability, an impedance-based protection principle is proposed. After defining the differential impedance and restraint impedance, the paper establishes a novel protection criterion according to the significant differences between the differential impedance and restraint impedance when normal operation, external fault or internal fault. Furthermore, the auxiliary criterion is constructed only by the amplitude characteristic of fault current to overcome the existing dead-zone of impedance-based protection. The proposed protection principle is verified by digital simulation. Compared with current differential protection, the impedance-based principle can effectively compromise the level of data synchronization and the band-width of data transmission. These characteristics play a significant role in adapting to the communication infrastructure of distribution network.