Abstract: Sub-synchronous dynamic instability is a major vibration fault of rotating machinery. According to general experience, sub-synchronous low-frequency vibration includes bearing instability (oil film whirl, oil film oscillation) and steam flow induced vibration fault. In this paper, through the analysis of the low-frequency vibration fault of the condenser, the Riccati transfer matrix method is used to calculate the natural frequency and stability of the shafting. According to the Hertz contact law, the dynamic response of the rub impact rotor is numerically simulated, the mechanism and fault characteristics of the sub-synchronous low-frequency vibration fault induced by the dynamic and static rub impact are studied, and the commonly considered bearing instability fault of the condenser is eliminated, It is determined that the dynamic and static rubbing is the main inducement of the low-frequency vibration of the condenser, guide the on-site maintenance, eliminate the rubbing fault, and restore the operation of the condenser to normal. It provides necessary reference for improving the understanding of sub-synchronous low-frequency vibration fault of condenser and safe operation of unit.