In this paper, the nondestructive testing and defect image signal recognition system for special welding of steel structure is researched and developed. The system not only makes use of the powerful computing power of the computer, integrates the relative software and hardware resources, but also realizes the functions of defect location, identification, display, channel control, storage and printing, etc., which greatly improves the cost performance of the instrument, and at the same time improves the intelligence of the instrument, making manual flaw detection more simple and flexible. The combination of modern digital signal processing and pattern recognition technology promotes the welding nondestructive testing technology, which not only improves the accuracy of testing, but also obtains the quantitative results of testing, such as the size, location and nature of welding defects.