The electric-drive-reconstructed onboard charger (EDROC) can effectively reduce costs and occupy the volume of charging equipment by reusing the drive system components into charging system and executing the electric drive and charging functions in a time-sharing manner. However, there is a risk of irreversible demagnetization of permanent magnet synchronous motors in EDROC systems at high temperatures. In view of this, this paper analyzes the degree of demagnetization of the motor under different conditions, and proposes the safe and stable operating conditions of the motor under charging conditions. Firstly, the winding currents of the EDROC system under DC charging mode and in-motion charging mode are analyzed. And then, the irreversible demagnetization model of the motor is established. Moreover, for different charging modes, detailed analysis is conducted on the demagnetization degree of the motor under different temperatures and charging currents. Based on this, the indispensable conditions for safe and stable operation of motor under charging conditions is established. Finally, experiments were carried out based on a 2 kW laboratory prototype to verify the accuracy of the irreversible demagnetization model and the necessity of the conditions for safe and stable operation of the EDROC system.