Based　on　the　low　frequency　electromagnetic　technology,　the　non-destructive　testing　of　the　internal　and　external　surface　of　the　ferromagnetic　structures　in　the　industrial　pressure　equipment　is　investigated.　By　analyzing　the　magnetic　flux　leakage　signal　and　the　amplitude　and　phase　change　of　the　magnetic　flux　leakage　signal,　a　method　for　quantitatively　characterizing　the　internal　and　external　surface　cracks　of　ferromagnetic　structures　is　presented.　Based　on　the　low　frequency　magnetic　flux　leakage　simulation　model,　the　variation　of　the　characteristic　parameters　of　the　magnetic　flux　leakage　signal　in　different　depths　and　different　positions　is　studied.　Furthermore,　a　single　channel　low　frequency　magnetic　flux　leakage　sensor　is　developed　and　applied　to　detect　the　internal　and　external　surface　of　the　ferromagnetic　structure　with　different　depths　and　different　positions　of　the　artificial　crack.　Simulating　and　experimental　results　show　that　the　amplitude　is　more　sensitive　to　the　depth　of　the　upper　surface　crack,　and　the　phase　value　is　more　sensitive　to　the　depth　of　the　lower　surface　crack,　which　provide　technical　support　for　the　safe　operation　of　the　ferromagnetic　structures　of　the　industrial　pressure　equipment.　©　2016,　Science　Press.　All　right　reserved.