Abstract: The weld formation, microstructure and mechanical properties evolution of pure copper MIG welded joints were studied with the preheating temperature increasing from 100℃to 400℃. The results show that with the increase of preheating temperature, the welding spatter decreases, the weld forming quality improves significantly and the weld penetration increases gradually. The microstructure of weld metal is mainly copper-based solid solution columnar grain structure, and the heat affected zone is equiaxed grain structure. The hardness of the welded joint shows the decreasing trend from base metal to weld metal. With the increase of preheating temperature, the width of weld zone and heat affected zone increases, the hardness of weld metal and heat affected zone decreases slightly, and the tensile strength of joint increases first and then decreases. When preheating at 300℃, the joint has the highest tensile strength and good weld forming quality.