The invention belongs to the technical field of industrial detection, and discloses a board defect area extraction method and device, and the method comprises the steps of obtaining a to-be-detected board image, and carrying out the segmentation of the image, and obtaining a plurality of detection units; obtaining gray threshold values in one-to-one correspondence with the multiple detection units; and calculating a gray average value of the detection unit, comparing the gray average value with a corresponding gray threshold value, and judging whether the detection unit contains defects or not. According to the invention, the to-be-detected board image is divided into the plurality of detection units, whether each detection unit contains defects or not is judged, the accuracy of the judgment result is improved, meanwhile, all the detection units containing the defects jointly form the defect area on the board; when the defects are further identified subsequently, only the detection units containing the defects are analyzed, so that the amount of data needing to be processed is greatly reduced, and the time required for detecting the defects of the board is shortened.