Abstract A mathematical model for flow simulation of full penetration laser beam welding of titanium alloy is presented. In this model, the heat source comprises a plane heat source on the top surface and a cylindrical heat source along the z-direction, which takes into account the plasma effect and the keyhole absorption. By solving the conservation equations of energy, momentum and mass, the temperature and flow fields are obtained. The momentum interpolation scheme with under-relaxation parameter is used to simplify the calculation algorithm and save the storage space of computer. The mushy region is introduced to provide a simple method to dispose of the pressure and velocity boundary conditions. Results calculated from the models are found to agree with the experimental results for the geometry profile of weld. The calculated results indicate the metal flow is the main reason for forming the typical “hourglass” cross-section profile.