The invention relates to a voltage transformer on-load tap-changer mechanical fault diagnosis method of which key technical points are that a vibration detection probe is adhered to a box wall of an on-load tap-changer in a surface-mounted manner, vibration signals generated in tap-changer operation processes can be captured, the vibration signals are subjected to energy frequency band decomposition operation via wavelet packet analysis technologies, frequency spectrum characteristics of the signals can be extracted, a state characteristic vector can be formed, a characteristic vector database is formed via signal collection and storage, an Euclidean distance function is used for analyzing a changing trend of the characteristic vector, and a mechanical state of the on-load tap-changer is diagnosed and assessed. Via the voltage transformer on-load tap-changer mechanical fault diagnosis method, the vibration signals generated in the tap-changer operation processes can be captured in real time, a characteristic quantity of the vibration signals is extracted and is compared with historical data, whether a fault occurs is determined and a fault development trend is determined, overhaul cost is lowered, detection efficiency is raised, early stage mechanical faults can be found timely and effectively, and the faults can be prevented from worsening.