The present invention provides a longest path preset protection P ring generating method based on circulation depth-first traversal. The method comprises the steps of simulating the topological graph of a communication network; obtaining the longest path r between the two given adjacent nodes, for example a node s and a node t, in the topological graph; adding a straight connection edge e between the node s and the node t in the longest path r to obtain a longest ring, namely the required preset protection P ring. The longest path preset protection P ring generating method based on the circulation depth-first traversal of the present invention applies a simplification method approximate to a hamiton loop method to the P ring search for the first time, so that a polynomial complexity of the longest path problem is obtained, the efficiency is improved, and the method can be applied to the large-scale communication network.