The present invention discloses an automatic exposure method basing on the target area in the image. The method comprises the following procedures: executing target detecting to the image of the frame being detected currently to judge whether a target exists; confirming the brightness value of the frame being detected currently according to the brightness value of the target area in the currentlydetected frame and/or the detected frame beforehand; and executing automatic exposure adjusting to the subsequent image according to the brightness value of the currently detected frame. The method adopting the invention can lead to that the target area in the image has appropriate brightness information relative to the full figure background. The signal-to-noise ratio of the target area is increase. The insufficient brightness or over exposure in the target area can be prevented. The effect is especially evident in the target backlight or bright light condition.