Abstract: The damage evolution of TC4 titanium alloy welded joint by CO₂ laser deep penetration welding was investigated with multi-specimens method.The research results show that the plastic damage evolution shows the process of the initiation,growth and coalescence of the micro-voids.The volume fraction of micro-voids nonlinearly distributed from the weld metal and HAZ to the base metal.The plastic damage evolution of the joint by laser welding has a relationship with the load.The plastic damage in weld metal is greater than that in base metal and HAZ at the lower load,but the volume fraction of micro-voids in HAZ will increase faster than that in weld metal with the load being increased,and the damage in the heat affected zone and base metal is greater than the weld when the load increases enough.The grain size in welded joint has little effects on the plastic damage,but the gradient of the grain size may delay the plastic damage of the welded joint by laser welding.