The invention discloses a transformer on-load tap changer mechanical state monitoring method and system. The method of the invention includes the following steps that: the vibration signal of an on-load tap changer during a switching process are acquired; phase space reconstruction is performed on the vibration signals, and delay time is calculated; coarse graining is performed on the vibration signals according to the delay time; a vibration signal matrix which is obtained after the coarse graining is performed is normalized; the base matrix of the normalized vibration signal matrix is calculated with the Lagrangian method; the low-dimensional coefficient matrix of the vibration signals and the statistical magnitude of the low-dimensional coefficient matrix are calculated; the elemental mean value of the statistical magnitude and the control limit value of the statistical magnitude are calculated; and the mechanical state of the tap-changer is judged based on the elemental mean valueof the statistical magnitude. With the method and system of the invention adopted, the mechanical state of the on-load tap changer of a transformer can be diagnosed, whether the mechanical state of the tap changer changes can be effectively and accurately detected, and therefore, effective measures can be taken in time for the tap changer, and the operational reliability of the tap changer can beimproved.