In order to study effect of defocus amount on the weld formation, the microstructure characteristics and properties of 5mm thick laser welding copper, metallographic microscope was used to analyze the microstructure and morphology of the weld. The tensile strength and electrical conductivity of the joint were tested. The results show that, when the absolute value of defocus is high, it is easy to spatter and cause holes on the weld surface. A well-formed weld can be obtained within the defocus range of 0mm~-2mm. The angle between the columnar crystal and the welding direction of the longitudinal section of the weld is gradually reduced from 90° on the sides to 0° in the middle. The columnar crystal near the upper surface of the weld is about 2.96 times the length of the columnar crystal near the bottom surface. The grain size of heat affected zone coarsen. And the width of heat affected zone corresponding to defocus +1mm is the largest. Under the penetration condition, the tensile strength of the joints corresponding to the different defocus amounts is equal, up to 77.3% of base metal. The joint elongation with defocus 0mm~-1mm is slightly higher than that with defocus -1mm~-4mm, up to 54.9% of base metal. The electrical conductivity of the weld is almost the same as that of the base metal. The study is beneficial to obtain good shaped laser welded joints of copper.