For　the　wind　turbine,　the　main　shaft　is　subject　to　complex　stress.　Cracks　are　easily　generated　and　expanded　from　the　outer　surface,　which　seriously　threaten　the　safety　of　the　wind　turbine.　In　order　to　monitor　the　main　shaft　in　service,　this　research　carries　out　an　on-line　monitoring　method　and　an　integrated　system　for　the　surface-opening　transverse　cracks　on　the　outer　circumference　of　the　main　shaft.　An　annular　array　of　ultrasonic　transducers　is　constructed　at　the　end　face　of　the　shaft　by　piezoelectric　elements.　The　parameters　of　the　array　are　determined　and　optimized　by　the　ultrasonic　field　characteristics　to　achieve　the　coverage　of　the　stress　concentration　region.　An　on-line　ultrasonic　monitoring　system　is　also　developed,　which　integrates　detection　terminals,　a　sensor　array　and　the　operating　software.　The　detection　terminal　is　composed　of　a　voltage　pulser,　a　receiver　and　a　multiplexing　unit.　The　detection　terminal　moves　synchronously　with　the　main　shaft,　dealing　with　multi-channel　excitation　and　signal　acquisition.　The　software　not　only　manages　the　entire　inspection　process,　but　also　achieves　defect　imaging　by　electronic　scanning.　The　experimental　results　show　that　5mm　depth　transverse　crack　can　be　examined,　and　the　defect　location　error　is　less　than　2.5%.　This　will　provide　an　effective　on-line　structure　health　monitoring　technique　for　wind　turbine　shaft.　©　APWSHM　2018.　All　rights　reserved.