Abstract: Aiming at the problems that the reactor interturn short circuit can not be perceived timely, this paper presents dry-type air-core reactor interturn short circuit model, and studies on-line monitoring and protection from the angle of the magnetic field. The method combining Ansoft software and hardware is employed to convert the magnetic signal to the electric signal. The influence of different positions of the detected coil on the reactor interturn short circuit fault is researched. Comparing the calculated result of detected coil voltage with the threshold amount, we can judge the status of the internal insulated structure of the reactor and estimate whether the fault will occur. Therefore, the misoperation of the protector can be avoided. Experimental data shows that the method using detected coil voltage to monitor the short circuit condition is accurate and feasible, the fault can be found in time and fault expansion can also be effectively avoided.