<p>This paper aims to study the sub-critical vibration behaviours of a synchronous condenser rotor system with considering the asymmetry of the rotor shaft and slot wedge effect. Based on the finite element method, a time-varying dynamic model of the SCR system is established and the transform matrix is adopted to reflect the coordinate system relationship between rotating and inertial frame. The rotor shaft asymmetry and the influences of slot wedge include material, the depth of the lunate and secondary groove on the sub-critical vibration are investigated. The sub-critical vibrations induced by asymmetric shaft can be found in present work. Aluminum slot wedges, shallower lunate grooves, and deeper secondary grooves result in larger stiffness differences and will enhance sub-critical vibrations of the entire system. It is of great significance to master the sub critical vibration characteristics of the synchronous condenser rotor system as well as the improvement of its structure design.</p>