The present invention provides a method for detecting a defect of a display panel and related defect detecting device. The method includes: utilizing lights having different colors to illuminate the display panel; obtaining a plurality of corresponding grey-scale diagrams when the lights illuminate the display panel; and determining whether the display panel has a defect. If the grey-scale diagrams indicate a grey-scale difference, determining that the display panel has a defect. In this way, the present invention is able to raise defect detecting ability for the display panel and prevent from missing the defects.