Abstract: The magnetic powder flaw detection is one kind of method used to inspect the near surface defect of hardware which is made up of steel and iron using the magnetic powder. In order to enhance the production efficiency, and liberate labor force, people applies the digital image processing technology in the magnetic powder test. This article introduced the automated fluorescent magnetic powder flaw detection system for the steel axis used in aviation which designed by author. Then, elaborated the application of digital image processing technology in the system from three aspects: image gathering, image processing and image analyzing. Author's algorithm can accurately and rapidly analyze the crack flaw's characteristic both qualitative and quantitative. It has achieved the anticipated goal.