Hydroxylated multi walled carbon nanotubes(MWCNTs) were selected as the surface activating flux to carry out the CO2 laser welding of TC4 titanium alloy. The penetration, width and ratio of depth to width of the weld joint were measured and calculated. And the microstructure, microhardness and phase composition of the weld joint were also studied. The results show that the coating formed with the solution of 4% MWCNTs is uniform and dense on the surface of TC4 titanium alloy. The presence of the coating can increase the penetration of weld joint by about 6% and reduce the width by about 4%. The ratio of depth to width is increased by about 10% under the condition of given welding parameters. It is suggesting that MWCNTs can increase the laser absorption. The β grain of the weld joint coated with MWCNTs is fine and α′ phase in the crystal is short because of the rise in welding temperature and cooling rate. In addition, MWCNTs didn't show an obvious effect on the microhardness and phase composition of the weld joint.