Abstract: With the increasing complexity of power grid structures, the setting of traditional transmission line backup protection becomes more and more difficult. In this paper, with the deployment strategy of phasor measurement units（PMUs） spaced apart for buses, the fault area is determined according to the wide-area differential method, and then the voltage and nodal injection current of the bus without PMUs in the fault area are estimated. The nodal voltage fault component equation in the area is derived,and the nodal fault-injection current is constructed. The change characteristics of nodal fault-injection currents of buses on both sides of the fault line and normal buses before and after the fault are analyzed, and the criterion of wide-area backup protection is constructed. Simulation results based on IEEE 39 bus system verify that the proposed algorithm is not affected by fault location,type and transition resistance, and can detect fault lines in various fault scenarios.