The invention discloses a method for on-line monitoring of a mechanical fault of an on-load tap-changer of a transformer. The method comprises the following steps of attaching three vibration sensorsto the top end of a box wall of the on-load tap-changer, and collecting a vibration signal generated in an action process of the on-load tap-changer; carrying out empirical mode decomposition (EMD) onthe collected signal, and computing an improved mask signal through a decomposed intrinsic mode function (IMF); adding the improved mask signal to the original collected vibration signal, and then carrying out empirical mode decomposition; and computing a power characteristic matrix of the intrinsic mode function according to an interval maximum power characteristic formula, comparing the power characteristic matrixes of the OLTC under different working conditions, and carrying out fault diagnosis on the OLTC. According to the method, a working state of the on-load tap-changer of the transformer can be monitored in real time, and the requirement for real-time fault diagnosis of the on-load tap-changer of the transformer can be met. A data support and a theoretical basis are provided for purposeful overhauling, and the waste of manpower, material resources and time is avoided.