In order to study the properties of laser butt welding joint of pure titanium sheet, a welding system consisting of fiber laser and robot was used to carry out the welding test. Effect of defocus on the properties of the joint was evaluated comprehensively by testing the tensile properties, porosity and cupping value of the joint. The microstructure of the joint was revealed. The results show that the tensile and forming properties of the joints are lower than those of the base metal. The defocus has no direct effect on the tensile properties. To a certain extent, the formability of the joint increases with the increase of defocus. When the defocus is +20mm, the joint has the best tensile and forming properties. The microstructures in the weld zone are coarse α grains + serrated α grains + a small amount of acicular α grains. The existence of acicular α grains is beneficial to improve the formability of the joints. The microstructure of heat affecting zone (HAZ) is composed of irregular coarse α grains + serrated α grains. The grain size of HAZ is lower than that of weld zone. This research has certain theoretical guiding significance for laser welding of pure titanium.