TC4 titanium alloy bottom-locking joint with a thickness of 3 mm was welded by disk laser. The type of porosity was judged by analyzing the pores characteristics. The suppress effects of porosity were studied by using three welding methods: pulsed laser welding, laser heat-conduction welding and defocused laser beam-high speed welding. The results show that a large number of porosities exist in the center of the weld, the porosity is irregular and large, and the surface of the porosity wall is rough, mainly for small pores. By using pulsed laser welding with lower welding speed and appropriate pulse frequency, the porosity is decreased to 0.5%～1%. By using laser heat-conduction welding with 0.4 m/min of welding speed, ＋15 mm of laser beam defocusing distance, the porosity is 0%, but the welding deformation is large and incomplete penetration defect is possibly formed. By using defocused laser beam-high speed welding, the porosity is obviously decreased with higher welding speed and longer laser beam defocusing distance. At the parameter of 2～3 m/min of welding speed, ＋5～＋7 mm of laser beam defocusing distance, the porosity is 0%～5%, and the porosity is significantly and stably suppressed.