Contactors are common electrical equipment widely used in many application fields, such as smart grid and intelligent electrical apparatus. To keep excellent conductivity and low contact resistance, the contact is generally made up of copper-based alloy. During the close process, electric arc may cause the welding of contact material to make the breaking of contactors more difficulty. In this study, several AC high-power vacuum contactors with copper-based contact are tested to explore the welding characteristics. The results show that the welding of copper-based contact is a complex and random phenomenon corresponding to several physical mechanism. A model is provided to describe the welding phenomenon to predict the reliable life of contactors.