The boundary fluctuation of the actual power grid will cause the low-frequency oscillation, and the severe oscillation of some lines under the main oscillation path will seriously threaten the security and stability of the power grid. Therefore, the paper proposes an identification method of the main oscillation path under the boundary disturbance of the power grid. By comparing the similarities between power system and elastic mechanical system, the power grid is mapped into an elastic network, and a dynamic model of elastic mechanics is constructed to analyze the low-frequency oscillation of power grid from the perspective of mechanics. According to the energy conversion mechanism of forced vibration, the energy conversion characteristics of power systems under power oscillation are analyzed. Combined with the interaction and distribution characteristics of the network energy, the path oscillation index is established, and an identification method of main oscillation paths is proposed. The simulation results show that the dynamic mapping between power system and elastic mechanical system is reasonable, and the identification of main oscillation path is effective. The presented method is helpful to understand the physical process of low-frequency oscillation in power systems, and provides references to the relevant measures to suppress oscillation.