For the identification of series resonance in power system branches, the identification method of branch series resonance based on port equivalent method is proposed. First of all, a typical three-bus test system is used to analyze the series resonance of the branch. It is found that the branch series resonance can be identified by the port equivalent admittance formed between the bus node and the reference node. Then, by establishing the node port association matrix and using the node impedance matrix, the multi-port Thevenin equivalent impedance matrix is obtained, in other words, the equivalent admittance matrix of the port is formed, and the load branch of the node is eliminated, as well as the effect of the node load branch on the series resonance of the identification branch is eliminated. The algorithm effectively utilizes the node admittance matrix commonly used in power system. Finally, the feasibility and correctness of the three-bus system and IEEE9 network system are proved by simulation analysis. The validity of this novel formulation is confirmed through theoretical analysis and case studies.