A novel wide-area back-up protection principle utilizing fault matching degree（FMD） is presented. During normal operation of the power system， the protection associated area（PAA） is formed based on the placement of phasor measurement units（PMUs） and the network topology. When fault occurs， by calculating the fault steady state component of differential current within each PAA， the fault associated area（FAA） is determined. The FMD therein is then calculated to precisely locate the fault. Verification by simulation with a 10-generator 39-bus system using distributed parameters shows that the method is free from the influence of the operation condition of the power system， and is capable of rapid， reliable， and sensitive fault location under the limited PMU measuring points condition. Furthermore， it is not necessary to modify the network matrix in the fault location process. The calculation burden is not heavy and the principle is easy to implement.