The invention discloses an active noise reducing device and an active noise reducing method for an air conditioner. The active noise reducing device comprises at least two noise acquiring devices, a master control module and a noise inverted phase signal output device, wherein the at least two noise acquiring devices are used for acquiring sound signals at different positions of an air outlet of the air conditioner; the master control module is used for analyzing the sound signals acquired by the noise acquiring devices, filtering signals which cannot be sensed by ears to obtain a noise signal, generating a control signal, and sending the control signal to the noise inverted phase signal output device; and the noise inverted phase signal output device is used for outputting a inverted phase signal to a noise source position, wherein the frequency of the inverted phase signal the same as that of the noise signal and the phase of the inverted phase signal is reversed to that of the noise signal. According to the active noise reducing device for the air conditioner disclosed by the invention, parts which generate noises are not needed to be distinguished, noises caused by all types and all possible reasons can be reduced, and the direction of the output noise inverted phase signal can be regulated at any time according to the detected noise source position, so that the problem that the direction of the noises is changed by regulation of an air feeding angle of an indoor unit and opening degree of an air deflector can be solved.