Abstract: The characteristic of oblique defect the Φ139~363 mm seamless steel pipe presents a 5°~10° oblique angle to the longitudinal direction of the pipe body, which leads to relative high difficult in detecting which once caused missed detection. Based on analysis and exploration of the NDT technology, it is finally determined to use the magnetic flux leakage inspection technology to detect the body oblique defect of the seamless steel tube. Accordingly a magnetic flux leakage inspection system especially used to detect the said defect is jointly researched and developed with relevant magnetic flux leakage detector-maker. As a result the said problem of difficult detecting the body oblique defect of the Φ139~363 mm seamless steel pipe is fixed.