A foreground image separation method is disclosed to separate dynamic foreground and static background in a sequence of input images which have been processed either with automatic white balance or brightness control by the camera. First, an input image is received from the camera, and then a white balance or brightness compensation is performed to the input image according to a reference image to generate a compensated image with background color and background brightness which are approximately similar to that of the reference image. Finally, a background subtraction algorithm is performed to the compensated image to generate a background separation result. The background subtraction algorithm could be a Gaussian Mixture Model based algorithm. The method could process successive images received from the camera to continuously generate background separation results and update the reference image accordingly, such that video surveillance system could adapt to the change of illumination.