Abstract: In order to detect the microcrack defect with 0.045 mm minimum depth located at the shell thickness of 0.65~1.2 mm, an orthogonal water immersion system with ultrasonic lamb wave testing was designed with data processing algorithm being based on helical scan mode. The system could achieve automatic quantitative testing result of zero undetected fault with low false call rate. Through multi-channel ultrasonic signal transmitting / receiving design and the feature extraction system, the system testing speed was of 6 s per piece.