Abstract: According to the features of laser deep penetration welding,three heat models of uniform heat source,double ellipsoidal heat source and the combined heat source were used to study numerically the weld bead geometry and residual stresses in CO₂ laser welding of TC4 titanium alloy.The results show that the heat source model in calculation should be selected according to the energy input.Then an approach to select the heat source model was proposed that the uniform heat source should be used under the condition of the greater energy input,the double ellipsodial heat source is suitable for medium energy input and the combined heat source fits for smaller energy input.Both the double ellipsodial heat source and the combined heat source can indicate the decrease of heat input with depth,and the estimated weld bead geometry as circular cone shape or nail shape.The residual stresses on top and back sides are similar when the uniform heat source is used,but the larger residual stress exists on the top surface than that on back surface when the double ellipsodial and combined heat sources are used.