Inter-turn short circuit fault is the main cause of dry-type air-core reactor faults. To quickly and accurately detect the inter-turn insulation condition of the reactor, a new method for online diagnosis of inter-turn short circuit faults in reactor coils is proposed. This method utilizes the terminal voltage and total current data of dry-type air-core reactors to construct characteristic graphs to reflect coil changes. The paper first studied the equivalent circuit models under normal and fault conditions and then established a two-dimensional simulation model based on the geometric parameters of the reactor. Finally, different inter-turn short-circuit fault scenarios were set up, and a series of verification analyses were carried out based on simulated voltage and current signals. The results show that this method exhibits significant variation patterns under different operating conditions of the reactor and has certain feasibility.