In the traditional dark channel defogging algorithm, there is blocking effect in the transmittance estimation, which is easy to cause the white edge phenomenon. At the same time, the dark channel principle is not applicable to the bright areas such as sky and white clouds in the image, and results in defogging image distortion.In this paper, a defogging algorithm based on multi-scale dark channel and adaptive tolerance is proposed, which can effectively avoid the above problems. Firstly, the rough transmittance estimates under three windows with different sizes are calculated, and the results are fused effectively. Then, the transmittance is refined by guided filtering to obtain a multi-scale transmittance map with better robustness and accuracy, and the adaptive tolerance strategy is introduced to correct the transmittance of the bright areas. Finally, the overall brightness of the defogged results is dark, so the brightness and contrast of the defogged image are compensated. Experimental results indicate that the information entropy increases by 0.2 bit/symbol, the average gradient increases by 0.5, and the PSNR increases by about 8 dB when different algorithms are used to the image without or with a small amount of sky. For processing more and a large number of sky region images, PSNR and SSIM are improved by about 3 dB and 0.1, respectively. The requirements of clear image details, reliable color and good defogging effect in bright areas are better realized.