The present invention discloses an automatic exposure method basing on the target area in the image. The method comprises the following procedures: (a) executing target detecting to the image of the currently detected frame to judge whether a target exists; (b) confirming the brightness value of the currently detected frame used for calculating the exposure parameter according to the brightness value of the target area in the currently detected frame and the brightness value of the target area in the previous detected frame when a target exists; and (c) calculating the exposure parameter according to the brightness value of the currently detected frame and executing automatic exposure adjusting to the subsequent image until the brightness of the last detected frame is confirmed and the exposure parameter is calculated. The method adopting the invention can facilitate that the target area in the image has appropriate brightness information relative to the total graph 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 brightlight condition.