The invention relates to methods and devices for generating a sound classifier and detecting an abnormal sound, and a monitoring system. The sound classifier comprises at least one classifier grade. For generating each sound classifier grade, the method for generating the sound classifier comprises the following steps: generating, based on an input sound sample, a normal sound model; calculating, based on the input sound sample and the normal sound model, a first threshold value, wherein the input sound sample is segmented to a first normal sound sample and a first abnormal sound sample according to the normal sound model and the first threshold value; generating, based on a specific abnormal sound sample and the first abnormal sound sample, an abnormal sound model; and calculating, based on the first normal sound sample and the abnormal sound model, a second threshold value, wherein the first normal sound sample is segmented to a second normal sound sample and a second abnormal sound sample according to the abnormal sound model and the second threshold value. The each classifier grade comprises the normal sound model, the first threshold value, the abnormal sound model and the second threshold value.