Background　US Food and drug administration approved the clinical application of BIS monitor for monitoring the pharmacodynamics of anesthetics and sedatives in October 1996. The application of BIS effectively reduced the occurrence of intraoperative awareness, and improved the postoperative recovery of patients and the perioperative safety. However, it was found that the values and changes of BIS were paradoxical in some clinical scenes.　Objective　To expound the reasons of the paradoxical changes of BIS values.　Content　The reasons might be related to many factors, including pharmacological characteristics and the effect site of anesthetics, interference, special pathophysiologic state (e.g. hypoglycemia, brain ischemia, low blood volume and neurological dysfunction), age, operation, and vasoactive drugs.　Trend　We still need to actively think about how to use BIS monitoring or combine it with other monitoring measures to improve the anesthetic management and the prognosis in particular cases.