Abstract: The active distribution network disconnection fault causes accidents such as overvoltage, personal injury and death, and motor damage. To solve this problem, we first analyzed the negative sequence impedance of each node after the active distribution network disconnection fault and the zero sequence voltage difference on both sides of the fracture. With the idea of current differential protection for reference, the negative sequence differential impedance was defined. Based on the characteristic difference of the negative sequence differential impedance when the fault occurs inside and outside the area, a protection method based on the negative sequence differential impedance was proposed. This method uses the amplitude of the negative sequence differential impedance as the amount of action, and uses its phase to construct an adaptive braking threshold. Secondly, in view of the failure of the overload protection, we proposed the zero sequence voltage amplitude difference as an auxiliary criterion. Finally, we analyzed the influence of the unmeasurable load branch on the above protection criterion, and proposed a method for fault location by comparing the magnitude of the negative sequence current. The simulation results show that the method can reliably locate fault and has the characteristics of not being affected by the neutral grounding method, transition resistance, load, and distributed power sources.