Background <p>Aero-engine casings with bolt-flange connections experience significant excitation forces from the rotor assembly, leading to complex dynamic behaviors at the connection interfaces. The nonlinearities at these interfaces, primarily caused by damping dissipation and variable boundary conditions, pose challenges for finite element (FE) methods in accurately simulating vibration characteristics of joined structures.</p> Methods <p>To bridge this gap, we propose a high-fidelity FE model for bolt-flange connections that accounts for the frictional characteristics of various connection interfaces and accurately captures their nonlinear mechanical behaviors. By combining nonlinear structural module with harmonic response module to directly generate amplitude-frequency response curves, this study provides a comprehensive analysis of dynamic softening behavior in conical-cylindrical shells with bolt-flange boundary.</p> Results <p>The accuracy of the dynamic model and the FE analysis procedure are validated through comparisons with experiments. Additionally, the study examines the effects of excitation level, bolt preload, and flange thickness on the nonlinear vibration characteristics, investigating the causes of dynamic softening in relation to contact stress and contact state at the connection interface. These findings offer valuable insights for optimizing bolted structures.</p>