The invention provides a method and system for monitoring an inter-turn insulation state of a reactor, and an application. The method is characterized in that a vibration simulation model of a power reactor coil is established, and a vibration signal of the reactor coil is acquired to obtain the corresponding coil stress, displacement and acceleration; variational mode decomposition is utilized todecompose vibration signals of the reactor coil under different states into multiple finite bandwidth eigenmode functions, and adaptive segmentation of the signal band is completed; the relative entropy of each finite bandwidth eigenmode function component and an original signal is calculated, the relative entropy is multiplied by a weight coefficient to obtain the weight divergence, and time-frequency distribution characteristics of mechanical vibration signals of the reactor coil are represented; the relationship between the weight divergence and the inter-turn insulation state of the reactor coil is established through the pattern identification algorithm, the relationship is optimized till the set conditions are met, and the current inter-turn insulation state of the reactor coil is outputted.