The Ti-6Al-4V alloy T-joints were prepared by double-sided synchronized laser beam welding with homologous filler wires. Influence of heat input on the stability of welding process, weld formation, microstructure, and mechanical properties of Ti-6Al-4V alloy T-joints was investigated through the high-speed camera during double-sided laser beam welding process. Results show that the heat input has a significant effect on molten pool behavior and droplet transition, which influences the appearance and welded quality of the T-joints. With the increase of heat input, the morphologies of welded T-joints change and the grain size increases. Martensite forms at the heat affected zone (HAZ) and fusion zone (FZ), resulting in the fact that the microhardness of HAZ and FZ is higher than that of the skin. Moreover, the tensile strength of welded T-joints along the directions of skin and stringer increases with increasing the welding heat input. The fractures occur at the base metal part of all the specimens, which is correlated with the martensitic strengthening effect of the joints.