TC17 titanium alloy was weld under different laser heat input conditions. Optical microscope, scanning electron microscope, transmission electron microscope, 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 in heat input, the morphology of weld changes from Y- to X-shaped. 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 composed of coarse columnar grains with strip dendrites inside, and the spacing of dendrite increases gradually with the increase in heat input. The heat affected zone comprises finer equiaxed grains, and the increase in heat input leads to the refinement of α phase and coarsening of β phase. Moreover, the TC17 laser welded joints all fracture at the weld zone in the tensile and fatigue tests. Under the influence of dendrite size, the tensile strength decreases with the increase in 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.