Electroencephalogram (EEG) is a non‑invasive brain activity measurement, which can reflect the function of the brain to a certain extent. This technique has been widely used for monitoring the depth of anesthesia. Due to the defects of EEG indicators in anesthesia monitoring, raw EEG research becomes a hot spot again. In recent years, EGG‑based anesthesia monitoring rapidly develops, and considerable efforts are made to find a more accurate, comprehensive, and convenient method for evaluating brain electrical activity. This article introduces the principles and characteristics of EEG spectrum analysis (spectrum analysis and time‑frequency analysis), functional connection analysis, and artificial intelligence (machine learning, etc), explores changes in brain state during anesthesia, and emphasizes the significance of EEG for anesthesia monitoring. This article reviews EEG analysis and its application for monitoring in the depth of anesthesia.