Abstract: In the qualification process of ultrasonic testing for austenitic stainless steel pipe weld, it is found that for the same outside axial defect, the length measurement results are greatly different while performing ultrasonic testing from outside or from inside for the same transverse surface defect. Through theoretical analysis and simulation test, the final demonstration is that the defect manufacturing process has an influence on the accuracy of the axial defect length measurement by ultrasonic inspection. Therefore, some optimization suggestions were put forward to optimize the implantable defect manufacturing process of the qualification block.