A rotor temperature monitoring method and system for a permanent magnet synchronous motor are provided. According to the method and system, an a-phase line current and a b-phase line current of a stator of a permanent magnet synchronous motor are obtained as a first line current and a second line current; further, a line voltage between the a-phase and the b-phase of the stator is obtained and a rotating speed of the rotor of the permanent magnet synchronous motor is obtained; and then, the first line current, the second line current, the line voltage, the rotating speed of the rotor, an inductance parameter of the permanent magnet synchronous motor and a temperature characteristic equation of a permanent magnet of the rotor are substituted into a preset rotor permanent magnet temperature expression to calculate and obtain the temperature of the rotor.