The change of current value caused by turn-to-turn short circuit of dry-type air-core reactor is solittle that failure detection is difficult to be carried out. In order to solve this problem，a new on-line monitoringsystem based on impedance variation of turn-to-turn short circuit is proposed. The numerical method is applied toanalyze the variation of equivalent resistance and equivalent reactance when dry-type air-core reactor winding shortcircuit happens in different places，and the monitoring method based on harmonic analysis method and quasi-synchronization sampling method is analyzed by theory. The hardware system，which takes single-chipmicrocomputer as the core of data processing and logic control，completes data acquisition of voltage signal andcurrent signal of the reactor. In the respect of software design，the impedance variation will be uploaded to the PCafter it has been calculated by using the above monitoring method，and then monitoring of turn-to-turn short circuitfault will be realized. Finally，the design of on-line monitoring system is studied by testing. The research resultshows that，the equivalent resistance increases and the equivalent reactance decreases when turn-to-turn shortcircuit occurs，and the variation of equivalent resistance is more obvious than equivalent reactance. The experimentresults prove that this monitoring method is true and the on-line monitoring system is feasible.