Abstract: Using spectrum window replace spectrum meter is a precondition to control droplet transfer with spectral information online.Two kinds of spectral window can be selected,that are window dominated by metal spectrum of the electrode and window dominated by shielding gas.Different spectrum windows reflect different information.Two spectral windows were selected and corresponding spectral information were obtained.This paper have given the experiments and analytical results of different spectral window signal,which show the window dominated by metal spectrum of electrode can precisely reflect the droplet transfer in welding arc because of the concentration of the metal vapor affected directly by droplet transfer and the window dominated by shielding gas line spectrum cannot reflect droplet transfer perfectly becasuce of the concentration of the shielding gas particle only affected indirectly by droplet transfer.It is wise to select the spectrum window where metal spectrum concentrates to detect the droplet transfer in welding arc.