The invention provides an abnormal behavior monitoring system comprising a first sensing unit, a second sensing unit, and a central control unit. The first sensing unit is used for sensing a monitoring area and generating a corresponding sensing signal. The second sensing unit is used for receiving the sensing signal, adjusting a background temperature threshold based on the sensing signal, detecting a temperature change of a main target body entering the monitoring area, and transmitting distribution data of the temperature change to the central control unit. The central control unit is used for analyzing the distribution data and thus identifying the behavior of the main target body. With the abnormal behavior monitoring system, the behavior of the main target body can be monitored effectively; and when the behavior of the main target body becomes abnormal, the related staff can be informed timely. The system can be applied in a wide area. In addition, the invention also provides an abnormal behavior monitoring method.