A　low　frequency　magnetic　flux　leakage　detection　technique　was　developed　for　the　detection　of　the　outer　wall　of　the　ferromagnetic　pipeline　with　heat　radiating　fins.　According　to　the　structural　characteristics　of　the　pipeline,　a　low　frequency　magnetic　flux　leakage　sensor　was　developed.　By　means　of　numerical　simulation　and　experiment,　the　detection　ability　of　the　sensor　was　studied　in　different　depths　and　widths　of　the　defect　in　the　coated　aluminum　pipe.　Results　show　that　the　amplitude　and　the　harmonic　distortion　rate　of　the　detected　signal　can　be　used　for　the　quantitative　detection　of　defect,　and　when　the　pipe　is　coated　with　aluminum,　the　intensity　of　signal　detection　of　defects　increases.　Therefore,　the　low　frequency　magnetic　flux　leakage　sensor　has　a　better　effect　on　the　outer　wall　damage　detection　of　the　coated　pipeline　in　the　actual　working　conditions.　©　2018,　Editorial　Department　of　Journal　of　Beijing　University　of　Technology.　All　right　reserved.