The invention discloses an oil-immersed transformer vibration on-line monitoring system with a built-in optical fiber grating sensor. An optical fiber grating acceleration sensor is arranged inside a transformer, the wavelength division/space division hybrid multiplex technology is utilized for forming an quasi distributed sensing network, the direct measurement on the three-dimensional vibration acceleration of a transformer body in the transformer operation process is realized, a new path is provided for the transformer vibration real-time on-line monitoring, an effective technological means is provided for the accurate evaluation on the transformer operation state, and the reliable support is provided for ensuring the safe and reliable operation of the transformer and realizing the intelligentization of the transformer.