A microcontroller receives color codes of pixel points in an image captured by an infrared camera. If there is a color code falling within a color code range corresponding to a warning temperature range, the microcontroller determines a location of the pixel point of the image corresponding to the color code. A block of a monitored area of the image corresponding to the location is determined. An alarm is activated and an LED (light-emitting diode) is controlled to point at the determined block to let monitoring people know which person object has an inappropriately high temperature.