Abstract: For features of fins crack initiation of spiral water cooled wall tube,a special miniature ultrasonic probe and ultrasonic testing instrument were developed.The confirmatory tests were carried out on specimen of spiral water cooled wall tube,which were engraved with the approximate fins weld crack and flat bottom holes.The confirmatory tests have realized detection of the double fins weld defect in the long and narrow space detection of the only 5 mm fin width,through the fins of the single surface coupling,which can be conveniently and effectively applied to the overhaul of spiral water cooled wall tube of ultra supercritical boiler.