Abstract: On-load tap-changer（OLTC） is a crucial equipment for regulating the voltage of transformers under load, and its reliability is of great significance to the safe and stable operation of power grid. Firstly, the typical structure and working principle are briefly introduced by taking double transition resistance OLTC for example. And then, three typical accident cases are analyzed, which indicates that mechanical fault is an important cause of OLTC failure. Finally, a mechanical fault model for OLTC of 110 kV transformer is established, and vibration analysis method is adopted to study the mechanical fault. The results show that the vibration signal changes significantly under different conditions, which provides important supports for the subsequent state detection and evaluation of OLTC.