The present invention discloses an on-line monitoring method and an on-line monitoring system for an axial pressing state of an electric power transformer winding. A natural frequency of an upper layer wire cake of the winding is extracted from vibration signals of an upper surface of a transformer oil tank according to a characteristic that the natural frequency of the upper layer wire cake of the winding reduces under the action of a stable component of an axial Ampere force, a degradation of the axial pressing state of the winding, namely the reduction of a pre-tightening force, results in the reduction of the natural frequency of the wire cake of the winding, and thereby on-line monitoring the axial pressing state of the large power transformer winding by following and observing a variation trend of the natural frequency. The on-line monitoring system includes a vibration signal acquisition module, a load current acquisition module, an analysis control module, a storage module, an output display module and a sound alarm module, wherein, the vibration signal acquisition module, the load current acquisition module, the storage module, the output display module and the sound alarm module are connected to the analysis control module respectively. The on-line monitoring method and the on-line monitoring system have the advantages of a distinct physical significance and a simple realization way.