Purpose <p>The widespread adoption of permanent magnet synchronous motor (PMSM) in new energy electric vehicle has led to the emergence of electromechanical coupling transmission system (ECTS). However, the ECTS is prone to torsional vibration during startup, braking and other transient conditions. The purpose of this study is to propose a torsional inerter-based nonlinear energy sink (INES) based on the elastic connected disc mechanism for suppressing the torsional vibration of the ECTS.</p> Methods <p>This paper establishes a dynamic model of the ECTS coupled with INES, accounting for both the nonlinearity in electromagnetic excitation and the nonlinear characteristic of the INES, and investigates its performance under various excitations. This study assesses the influence of PMSM and INES structural parameters on the ECTS dynamics and optimizes the INES structural parameters using a genetic algorithm.</p> Results <p>Using the INES results in smaller torsional angle peak amplitude and broader stable frequency region under harmonic excitation. Using the INES leads to smaller torsional angle amplitude and shorter vibration attenuation time under shock excitation. Using the INES decreases the root mean square (RMS) value of relative torsional angle between the transmission system and PMSM under random excitation. The optimized INES further reduces the RMS values of relative torsional angle and improves the system stability.</p> Conclusion <p>The proposed torsional INES combines the mass amplification property of the inerter and the vibration reduction effect of NES, it can enhance the dynamic performance of the ECTS and also meet the lightweight design requirement in the transmission system.</p>