Video surveillance or laser imaging is widely used in atmospheric environment monitoring, but the image quality is inevitably reduced in fog weather. In order to meet the need of contrast enhancement and noise suppression in foggy image, an improved image enhancement algorithm for foggy image is proposed. Firstly, the original image is enhanced by bidirectional histogram equalization processing and fusion. Then the same method is used to deal with the low-frequency coefficient matrix which obtained by wavelet decomposition of the last fused image. Finally the output image can be obtained by wavelet reconstruction of the processing result and the high frequency coefficient matrix. Experimental results show that the proposed algorithm can achieve better contrast enhancement and noise suppression performance in fog surveillance images.