Abstract: Welding arc spectrur reflects all sorts of variation of the arc.Different spectrum band and different spectral line of spectrum contain different information.Droplet transfer directly leads to the variation of particle density and distribution of the arc.Terefore,detecting the intensity variation of correspond spectrum can detect the droplet transfer in welding arc.A general map of spectral line distribution characteristic is described in this paper.On the bases of analyzing of spectrum window radiation theories,the principle of spectrum window selection is put forward.According to the principle of selecting a specific spectrum window of droplet transfer mentioned above,a specific spectral window is selected and is used to detect and control droplet transfer on line.Experiment results show that the specific spectral signal from the spectrum window can reflect droplet transfer as sensitive as the signal of spectral ling from spectral meter.The droplet transfer detection and loop control with the specific window spectrum has achieved great success.