Abstract: Aiming at the requirement of online ultrasonic testing for large scale semifinishing workpiece and considering the influence of its surface contour error(such as out-of-roundness, conicity) on stabilization of the clearance between the probe and the workpiece, the design plan of adaptive surface shape tracking scanner based on ameliorated Cardan device was advanced and the mechanism analysis of the scanner was performed, and a novel scanner whose posture was adaptable to the specific partial condition of the large scale rotational part was developed. The scanner made the ultrasonic wave transmit into the rotational part along with the normal direction of the detecting surface so as to realize online ultrasonic flaw detection for large scale rotational parts. By the combination of the scanner and the large scale rotor online ultrasonic detection system, a software system was developed, which realized the requirement of online ultrasonic testing process and machine process run synchronously. The practical application proved the developed system elevated the ultrasonic testing quality and efficiency of large scale rotational parts.