The new graph theory algorithm is sys-tematically proposed in the paper. The new algoithmincludes the following main aspects:(1) On the basis of graph theory,exact definitionof simple loop is given and a new method to enumerateall simple loops is presented,the method ensure cor-rect enumeration of all simple loops. (2)The concept of phantom branch is put forwardto substitute the concept of phantom node,which isused in existing graph algorithm. The new concept ofphantom branch may settle the protection coordinationof the multiple-teed feeders. (3)The relationship of primary-back-up(P/B re-lationship)is put forward to substitute the concept ofexisting primary-back-up protection pair,and amethod to decide the P/B relationship from incidentmatrix is described as well.Some work examples are given,these examples provedthe effectiveness and correctness of the algorithm.