A wide area backup protection algorithm based on correlation matrix of line protection is proposed in order to realize protection with simple，reliable and high fault-tolerance performance. Direction information and distance information of local line as well as direction information of adjacent lines are taken into account. The correlation matrix of transmission lines protection is derived with the line association factor to implement the algorithm. It can identify the faulty component by calculating sum of each row elements values in the correlation matrix and comparing with threshold value. Both enumeration method and Monte-Carlo method are respectively used to study the fault-tolerance performance of the proposed algorithm for IEEE5-generator14-bus system as an example. The results show that the proposed algorithm is simple and reliable，and has good fault-tolerance performance.