Further　study　was　needed　to　solve　the　problems　about　the　spreading　complexity　of　low-frequency　ultrasonic　guided　wave　longitudinal　mode　L　(0,　2)　along　the　bend　pipe　and　the　difficulty　of　its　defect　detection　of　typical　parts　of　bend＇s　extrados.　Numerical　simulation　method　was　introduced　to　analyse　spreading　characteristic　of　mode　L　(0,　2)　in　extrados　of　five　steel　bend　pipes　with　different　curve　radius,　in　order　to　research　energy　distribution　through　snapshot　and　mode　transition　through　time　domain　waveform.　Defect　detection　and　mode　transition　of　mode　L　(0,　2)　were　studied,　through　experiments　of　eight　different　bend　pipes　with　circumferential　defects　in　lateral　and　medial　of　bend＇s　extrados　and　in　the　straight　part　after　bend.　The　results　show　that　the　energy　of　mode　L　(0,　2)　was　focused　on　lateral　part　of　bend＇s　extrados　where　the　circumferential　defect　could　be　easily　detected,　but　the　opposite　situation　occured　in　medial　of　bend＇s　extrados;　and　obvious　waveform　transformation　occured　when　mode　L　(0,　2)　spreaded　along　straight　pipe　after　bend　and　the　defect　in　straight　pipe.　In　conclusion,　this　research　provides　a　theoretical　guidance　for　engineering　applications　of　pipe＇s　defect　detection　using　low-frequency　ultrasonic　guided　wave.