Abstract: The superconducting joint of the internal coil in the vacuum coil system of the device has a high requirement on weld quality because of its bad working environment. In order to verify the feasibility of welding oxygen free copper pipe with arc preheating method, the method of arc preheating with open head was adopted to preheat oxygen free copper pipe to reach the temperature required for welding copper pipe . Non-destructive testing , macroscopic and microscopic examination, and hardness measurement of weld, heat-affected zone and base metal are performed on copper welded joints. The influence of arc preheating on the heat-affected zone is evaluated. The analysis shows that there is no significant difference between the hardness of the heat affected zone and the base metal, but there is a significant difference in the microstructure. The hardness of weld metal is slightly higher.In the observation range, arc preheating has no significant effect on the heat-affected zone. The results show that arc preheating is feasible.