Abstract: Aiming at the problem of frequent occurrence of inter-turn short circuit fault of dry-type air-core reactor, by analyzing the changes of state parameters during the inter-turn short circuit of 35 k V dry-type air-core shunt reactor, a fault feature extraction and detection method of dry-type air-core reactor based on wavelet packet decomposition is proposed. Firstly, the circuit model of the reactor under normal and fault conditions（inter-turn short circuit of the same strand and inter-turn short circuit of different strands）is established. The calculation methods of each electrical parameter are given, and the MATLAB/Simulink simulation circuit is built. The wavelet packet decomposition method is used to analyze the variation law of the total current flowing through the reactor under normal and fault conditions. Secondly, according to the decomposition results, the energy characteristics of each frequency band node signal are extracted and analyzed and by comparing the differences of node signals in the same frequency band, the operation state of the reactor is judged. Finally, in view of the problem of misjudgment caused by unclear initial electrical quantity changes during the inter-turn short circuit of the reactor, the entropy weight method is used to weight and calculate the difference of the average current value and the cosine similarity, which improves the accuracy of fault detection. The experimental results show that the wavelet packet decomposition of the current under the fault condition can accurately identify the inter-turn short circuit fault, which can provide a theoretical reference for the operation of the dry-type air-core reactor.