Abstract: For the flexible-node electric porcelain high-voltage equipments with concentrated and distributed parameters, the frequency equation is presented by the vibration theory of beam with distributed parameters. Introducing the boundary conditions of concentrated parameters, the frequencies and mode shapes can be obtained by the numerical method. According to the Betti law, the orthogonal conditions of modes of high-voltage equipment with concentrated and distributed parameters are derived. These orthogonal conditions can be used to decouple vibration equation of concentrated and distributed parameters, thus to obtain the generalized mass and stiffness. Therefore the responses of structure under earthquake excitation can be solved by the mode superposition method. In order to validate the correctness of this semi-analytical method, the responses of the 550kV metal oxide lightning arrester under earthquake excitation are solved by the semi-analytical method and finite element method. Result comparison shows the responses obtained by two methods are basically consistent, which indicates the correctness of this semi-analytical method. Therefore one new way is provided for seismic response analysis and seismic design of the flexible-node electric porcelain high-voltage equipments with concentrated and distributed parameters.