With the large-scale access of distributed generation (DG) and the continuous increase in power requirements, active distribution network (ADN) with active control, coordination and management capabilities has become one of the development trends of power system. However, due to the complex structure and operating mode, the occurrence of break fault greatly reduces the value of ADN, which seriously threatens the safety of equipment and people. Due to the limited influence of break fault on the traditional distribution network, the present break fault protection technology is insufficient. Especially in ADN, due to the influence of DG and the flexible and adjustable topology, accurate identification and protection of break fault are still difficult to achieve. For this reason, this paper analyzes the characteristics of ADN and its influence on break fault protection. And the variation characteristics of sequence current of fault feeder and non-fault feeder under single - phase and two - phase break fault are analyzed. On this basis, an active distribution network fault protection method based on negative sequence current ratio is proposed, which takes the negative sequence current ratio as the protection characteristic parameter. The theoretical and simulation results show that the method is not affected by load distribution and fault location, and is suitable for various network topologies, DG capacity and neutral grounding modes, with high applicability and sensitivity.