The utility model relates to an on-line monitoring and protection device of a series reactor for preventing accidents of coil ignition and burning and the like caused by turn-to-turn short circuit of the reactor, and belongs to the technical field of on-line monitoring and protection of the reactor. The technical scheme is that the on-line monitoring and protection device of the series reactor comprises a reactor (2), a detection coil (3), an insulation transformer (4), a filter module (5), a data collecting module (8) and a control computer (9), wherein the detection coil (3) is arranged on the reactor; the output of the detection coil is connected with the insulation transformer; the output of the insulation transformer is connected with the filter module; the output of the filter module is connected with the data collecting module; and the output of the data collecting module is connected with the control computer. According to the utility model, the effective monitoring of turn-to-turn short circuit accident of the reactor can be realized, the loop of the reactor in an accident is cut off immediately under the unmanned condition, the system and other devices are protected from being influenced, and the on-line monitoring and protection device of the series reactor has the functions of preventing accident extensification and the like caused by short circuit ignition.