Abstract: Arc spectrum information is a rising information source after arc electric information.It has important significance in developing automatic control technique of welding process.Its basic theory and method play an important role in expounding and applying arc spectrum information.This paper sets up a system of equations including 12 equations using existed plasma physics theory and spectrum theory,which play a role of basic theory of arc spectrum information.A basic view of arc spectrum information that arc spectrum information is a reflection of arc state and abundant information resource was drawn through analyzing the 12 equations.From the basic theory,basic methods of measurement and control also can be drawn.The application direction of basic methods of measurement and control be pointed out.