Due to operation error of electrical equipment, insulation and aging of motors, electrical appliances, wires and other electrical equipment, mechanical damage, combat damage and other reasons, power lines are likely to encounter short-circuit or overload fault. In the annular distribution system, bus bar short-circuit fault will do fatal harm to aircraft, to improve the reliability of distribution system we have to protect the ring bus bars, and effectively isolate the fault bus bar. Simulating the fault system relay on EMTP, analyzing the variation of fault can meet the requirements that protection devices do not refuse operating and backup protect devices do not acting error. According to inrush current and voltage drop of the fault system, we can set protection setting value and criterion of starting, and do related simulation test relying on Matlab. It proved that the new method can achieve automatic isolation of fault bus bar, ensures the safety of aircraft electrical power distribution system, and it has importance to improve aircraft power distribution reliability.