The invention relates to a looseness detection method of a bolt, and belongs to the technical field of mechanical state detection and maintenance. The looseness detection method comprises the following steps of first marking a tightened bolt to be detected, after the bolt to be detected is loose, and taking a picture, which includes a first mark, a second mark and the complete image of the bolt to be tested, of the bolt; after the image is taken, recognizing the coordinate positions of the first mark and the second mark in the picture respectively through image graying, image binaryzation, edge detection and characteristic symbol judgment by using the angle calculation method of the image; calculating a relative rotation angle between the two marks according to the coordinate positions; comparing the relative rotation angle with a set angle threshold value, and carrying out a tightening operation on the loose bolt. By using the looseness detection method, the pertinence of the looseness detection of the bolt is improved, and the safe and reliable running of equipment is guaranteed.