To advance the visibility of the fog-degraded image, a method for fog-degraded image clearness was presented. To restrain the noise amplification in sky region，the sky region first was segmented before enhancement, then using optimal normal distribution to put the pixels of the high grey region into that of the fuzzy，the range of pixel value in the sky can be got. By the information，the sky region can be segmented from the image. A moving mask was adopted to segment scene at different depth，then each pixel in the mask is processed using block-overlapped bi-histogram equalization. The experimental results show that this method can efficiently improve the degradation of image and enhance the clearness of image.