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.