Seismic safety of the converter valve is crucial for the safe operation of the entire flexible DC transmission system. In order to enhance the seismic resistance of the supported converter valve, composite diagonal rod insulators were designed to be added between the support insulators, which have a nonlinear property that can only be subjected to tension but not compression. Using finite element method, modal analysis of the valve structure was first conducted to study the impact of diagonal rod insulators on the dynamic characteristics of the valve structure. Secondly, artificial seismic waves were input for dynamic time history analysis, and the differences in dynamic response between two valve structure models with nonlinear and linear characteristics of diagonal rod insulators were compared and analyzed. Finally, according to the current specification requirements, three seismic waves were input to conduct seismic safety analysis and evaluation of the valve structure with nonlinear property of diagonal rod insulators. The results indicate that the acceleration and stress response results of the nonlinear valve structure model are greater than those of the linear model, indicating that using a linear model for seismic safety evaluation is more risky. Therefore, it is necessary to consider the nonlinear characteristics of diagonal rod insulators for seismic performance analysis of valve structure.