Abstract: The weld inspection of nuclear facilities has a short limit time and high acceptance requirements. At the same time, traditional film radiography technology requires a special time window, cannot cross operation and has high radiation risks. To solve these problems, this paper innovatively proposed to use alternating current field measurement (ACFM) technique to detect small defects of about 0.5 mm in thin-walled stainless steel welds. The feasibility of applying this technology to the detection of thin-walled stainless steel welds was studied through theoretical calculation; the probe design was optimized by simulation calculation to improve the detection accuracy and stability of ACFM. The verification results showed that for thin-walled stainless steel welds, this technology can detect all the more harmful area defects, and the volume defect with a diameter of about 0.4 mm can be detected.