Abstract: An unified model of gas phase area, liquid phase area and solid phase area during the molten pool flow process of back reflection induced synergistic laser welding was established. Plasma & vapor cloud and keyhole absorption were considered and effects of surface tension, thermal buoyancy and gravity were taken into account in the model. The three-phase keyhole phase change and flow field of molten pool were obtained by numerical calculation, surface tension effect on flow and heat transfer of molten pool was mainly discussed. Moreover, a validation experiment of back reflection induced synergistic laser welding was carried out by taking titanium alloy sheet as an example. Results show that the coupling effect of plasma & vapor cloud caused by keyhole and back plume induced by back sub-plate is the main reason for the formation of X-type weld, and surface tension is the main driving force of the “oval reflux ring” formation in the back reflection synergistic laser welding molten pool.