Abstract: The droplet transfer mode and arc length variation are the significant factors affecting the radiation intensity of arc plasma for MIG welding. It is critical to understand the regularity of the radiation fluctuation in application in industry control of the welding process. Therefore,the spectrum of MIG arc radiation was collected with a spectrometer based system. The spectrums of welding processes at different welding parameters were analyzed to study the regularity of the radiation variation. Based on the arc physics,the spectral information combining with the droplet transfer mode was used to get a better understanding of the arc radiation. The results show that the arc spectrum with its distinct distribution is different at different welding parameters. The MIG arc emits wavy radiation due to droplet transfer. The spectral signals in different spectrum bands,such as ultraviolet, visible and near infrared band,have totally different variation characteristics during different instance of the droplet transfer.