The invention discloses an on-line monitoring device for magnetic steel's loss of excitation for an elevator traction machine and a monitoring method. The monitoring device comprises a master controller of the traction machine and a permanent magnet synchronous motor electrically connected with the master controller of the traction machine. Operator contactors and a star-delta contactor are arranged on a main circuit between the master controller of the traction machine and the permanent magnet synchronous motor. The edge of a stator winding of the permanent magnet synchronous motor is provided with a temperature sensor which is in signal connection with the master controller of the traction machine. The on-line monitoring device for magnetic steel's loss of excitation for the elevator traction machine has following beneficial effects: the on-line monitoring device is capable of achieving on-line monitoring of the risk of magnetic steel's loss of excitation on a star-delta state of the elevator traction machine; major elevator accidents caused by magnetic steel's loss of excitation are avoided; and safety performance of an elevator system is enhanced.