Adaptive active noise control system with unstable state handling and associated method

An adaptive active noise control (ANC) system includes an ANC circuit and a control circuit. The ANC circuit generates an anti-noise signal for noise reduction, wherein the ANC circuit includes at least one adaptive filter. The control circuit receives a first input signal derived from a reference signal output by a reference microphone that picks up ambient noise, receives a second input signal derived from an error signal output by an error microphone that picks up remnant noise resulting from the noise reduction, and performs a transfer function variation detection based on the first input signal and the second input signal to control the at least one adaptive filter.