Welding arc is full of spectral information. Studying and utilizing the information to diagnose and control the welding arc is the most effective way to develop and improve the welding technology. Furthermore, it can be used to study and control other relative physical phenomena (such as gaseous plasma, laser processing plasma, underwater plasma arc, etc.). It is of great theoretical and practical significance. This paper has summarized the developing situation of the research on this field, expounded the principle of measurement and control by welding arc spectral information, described the application situation of welding arc spectral information, for example, the arc diagnosis, monitoring of arc atmosphere, imaging of arc welding region and control of droplet transfer in pulsed MIG welding, etc. At the same time, the authors have also introduced the application of arc spectral information in the development of plasma technology, such as in the development of plasma torch with carbon electrode and in the judgement of local thermodynamic equilibrium of underwater plasma arc. At last the authors have put forward some points of basic view on the technology and pointed out its developing direction.