Abstract: The 2 mm thick T2 copper was first non-preheated and preheated at different temperatures (100, 150, 200 ℃) then welded by fiber laser. The influences of preheating before welding on the microstructures and properties of the welded joint were investigated.The results show that the fusion depth and width of the welded joint with preheating at 150 ℃ were larger than those of without preheating and the equiaxed crystals in weld and cellular grains in fusion zone were coarser. The weld resistivity of the welded joint with preheating decreased comparing to those without preheating, indicating that the preheating improved the conductivity of the welded joint. The micro-hardness of the welded joint with preheating at 150 ℃ was higher than that of at 100, 200 ℃. The micro-hardness first decreased then increased with the increasing distance from the weld center and that of weld center was the largest. With the preheating temperature increasing, the free corrosion current density of the weld first increased then decreased. The corrosion resistance first decreased then increased while the difference was very small.