Welding is one of the very common process in industrial production. It is a kind of manufacturing process and technology which joint mental or other thermoplastic materials by heating, high temperature or high pressure. Welding quality will directly affect the final quality of workpiece. So during the welding process, each links have strict standard and welding quality evaluation has different indicators. Therefore, how to make a rapid detection to weld defect has become a valuable research. This topic is aimed at weld defect detection of small workpiece. The study contains the selection of sensor, design of detection system, hardware platform, software design, user interface design, etc. In the end, a set of high accuracy detector of weld defect will be designed.