In order to solve the problems of setting and coordination of the traditional back-up protection and its malfunction when CT breaks, a wide area back-up protection algorithm based on the correlation of sample value is proposed. The algorithm makes full use of the voltage and current information of the power grid, and the protection criterion is constituted based on the correlation of the sample value of the associated IEDs. Associated IEDs is searched through the associated matrix when CT breaks. The fault element is identified according to the correlation of the associated IEDs and the correlation of the virtual CT. This algorithm is still effective when synch precision is low and CT breaks. Finally, an IEEE 3-machine and 9-bus system is constructed by PSCAD, the results of the correlation degree of sample value show that the proposed algorithm can identify various types of faults on line and bus, and has good operational performance.