In order to understand the effects of nonlinear microstructures on the damage evolution in laser welded joint of titanium alloy, a multi-specimen method with volume fraction of microvoids was applied to estimate the damage evolution for TC4 titanium alloy. The results show that there are great nonlinear gradient distributions of the microstructure morphology in TC4 laser deep penetration welded joints from weld metal, HAZ to base metal, the damage performance and mechanism change according to these gradient characters, the damage mechanism presents the pattern of microvoids-polymerization, and the volume fraction of microvoids has a great gradient distribution which increases faster in the base metal near welding HAZ than in the weld metal as the load increasing.