The invention discloses a sonar image-based underwater structure crack extraction method. The method is characterized by comprising the following steps of: 1, obtaining an underwater sonar image of an underwater structure with cracks; 2, carrying out overlap-free image block division on the obtained sonar image to obtain a plurality of image blocks; 3, calculating an average gray-value, a standard deviation and an entropy of each image block, and mapping the average gray-values, the standard deviations and the entropies to a three-dimensional space; 4, dividing the image blocks into two types by adopting a K mean value clustering analysis method in the three-dimensional space: image blocks which possibly contain crack information and image blocks which do not contain crack information, and removing the image blocks which do not contain crack information; 5, denoising the image blocks which possibly contain crack information by adopting tensor voting, and connecting crack fragments; and 6, obtaining a final result by using a threshold value segmentation method. According to the sonar image-based underwater structure crack extraction method, scientific basis is provided for estimating the health conditions of underwater structures, and the detection correctness is improved.