The formation mechanism of typical welding defects in copper and steel dissimilar metals was studied using fiber laser. Results show that the main welding defects in laser welded of copper steel dissimilar metals were micro-segregation, porosity and solidification cracking. The main reason for the micro-segregation and solidification cracking was the composition segregation during the solidification process of molten pool metal. The Cu element segregation at grain boundaries weakened the bonding force between grains, thereby resulting in offsetting the thermal stress during the welding process by virtue of the plastic deformation of grains and the occurrence of grain boundary cracking. The formation of pores was mainly related to the keyhole instability during the laser welding process. The keyhole instability caused the collapse of the inner wall of the keyhole, which resulted in that the metal vapor cannot overflow smoothly from the molten pool surface. The method of alternating magnetic field assisted laser welding was proposed to restrain the defects of welds and obtain good weld formation.