The T-joints of Ti-6Al-4V alloy are fabricated by double-sided synchronized laser beam welding with the homologous filler wire. The influence of the welding conditions on weld defects such as lack of fusion and porosity are investigated. Results show that the lack of fusion defects are determined by the laser power, welding speed, laser incident angle and position. With larger enough welding heat input (higher laser power, and slower welding speed), the lower laser incident angle and higher laser incident position are required to avoid the lack of fusion defects. The quantity of plasma/metal vapor above the molten pool for weld seam with the lack of fusion defects is larger than that of the weld seam without the defects under a certain laser power. The porosity defects in the welded T-joints are mainly the type of keyhole porosity defects under different welding conditions, and the higher welding speed is helpful to decrease the porosity defects under a certain laser power.