This paper puts forward an ultrasonic detecting method of lamb wave according to the distribution feature of tiny(0.05 mm)internal flaws in the thin-walled(thickness below 1.2 mm)cylinder.Linear quantitative detection of different depth of flaws in the thin-walled cylindrical pipe has been carried out by choosing energy distribution of lamb wave.An automatic on-line detecting system with high speed

high sensitivity and low wrong judgment has been designed

which can automatically identify and select multi-type product.