The invention discloses a field failure on-line diagnostic method and system for a permanent magnet synchronous motor. The field failure on-line diagnostic method comprises the steps of (1) sampling a stator three-phase current signal of the motor to be detected, (2) determining a field failure characteristic frequency according to the number of poles of the permanent magnet synchronous motor to be detected, using the field failure characteristic frequency as a field failure characteristic quantity, (3) estimating the frequencies of frequency components in the stator three-phase current signal to be detected, judging whether a field failure happens or not according to whether the field failure characteristic quantity appears or not, and (4) after estimating the amplitudes of the frequency components in the stator three-phase current signal to be detected, calculating the ratios of the amplitudes of the frequency components to the amplitudes of same-frequency components within a same-current range of a motor in the normal state so as to judge the degree of severity of a field failure. The field failure on-line diagnostic system comprises the permanent magnet synchronous motor, a Hall current sensor, a signal conditioning circuit, a data acquisition circuit, a diagnosis control circuit and a display circuit. The field failure on-line diagnostic system is short in sampling time and high in anti-jamming capability, can judge field failures at different rotating speeds and under different load conditions, and is applicable to dynamic on-line monitoring.