In this paper, different laser heat input conditions were designed to weld TC17 titanium alloy. Optical microscopy, scanning electron microscopy, transmission electron microscopy, tensile and fatigue tests were used to compare the macroscopic morphologies, microstructures, and mechanical properties of the welded joints. The results show that with the increase of heat input, the morphology of weld changes from Y to X type. The number of pore defects in the weld increases first and then decreases. The pore defects are mainly distributed in the middle and lower part of the weld zone. The weld is comprised of coarse columnar grains with strip dendrites inside, and the spacing of dendrite increases gradually with the increase of heat input. The heat affected zone comprises finer equiaxed grains, and the increase of heat input leads to the refinement of α phase and coarsening of β phase. Moreover, the TC17 laser welded joints all fractured in the weld zone in the mechanical tests. Under the influence of dendrite size, the tensile strength decreases with the increase of heat input. The welding pore is the main reason for the fatigue fracture, and the fatigue life peaks when the number of pore defect is the lowest.