The visual document image is the electronic image about newspapers, books or magazines taken by the digital camera, the digital vidicon etc. Whose getting is more convenient than got from the scanner. Along with the development of OCR technology, visual document images could be recognized by OCR. Affected by some factors, digital image will be degraded during its acquisition, processing, transmission. One of the main problems affecting image quality, leading to unpleasant pictures, comes from improper exposure to light. So preprocessing is becoming much more significant before recognition in order to get an appropriate image satisfied recognition requirements. For the low contrast images with underexposure, according to the visual document image's characteristic, a new algorithm, based on image background separation, for image object enhance is proposed, The proposed method calculate the threshold of separation firstly, And different processing be taken on foreground and background: Various gray values in image background will be merged into unitary gray value, whereas the contrast of foreground will be enhanced. The proposed algorithm implemented in Visual C++ 6.0, and compared the result of proposed algorithm with the results of Otsu's method and histogram equalization. The experimental results show clearly that this algorithm could enhance the details of image object adequately, increase the recognition rate, and avoid the block effect at the same time.