Abstract: A surface defect detection system for branch pipe fillet welds was developed based on alternating current field detection (ACFM) technology. A hardware detection system including ACFM probe and portable chassis was designed, and a signal smoothing filtering algorithm was developed based on the characteristics of the detection signal to improve the signal-to-noise ratio. Based on LabVIEW software, a detection software was developed to integrate signal acquisition, signal processing, and signal display. Finally, a complete AC electromagnetic field branch corner weld surface defect detection system was formed, and the Q345 material corner weld specimen was tested using this equipment. The experimental results showed that the proposed alternating current field defect detection system can effectively reduce noise and accurately identify surface defect signals of branch pipe corner welds.