A novel high speed tandem TIG welding process was introduced to join pure titanium thin plates with high effiffifficiency and high quality. In order to reveal coupled mechanisms of arc, weld pool and microstruc ture in this novel welding process, coupled electrode, arc and weld pool numerical models were developed for both the single TIG welding and high speed tandem TIG welding processes, to study the arc and weld pool behaviors in a similar line energy input condition. In the high speed tandem TIG welding, the arc interaction causes expansions of arcs, and decreases of current densities; therefore, the maximum arc temperature, arcvelocity, and energy flfluxes on the top weld pool surface decrease to the values similar to these in the single TIG welding. Owing to a “Pull-Push” flflow pattern caused by the Marangoni force and arc shear stress on the top weld pool surface between two arcs, the flfluid flflow and energy propagation of the weld pool become much stronger. The existence time of high temperature is shorter, and the stirring effect of the weld pool is stronger, so fifiner weld microstructures can be obtained in the high speed tandem TIG welding.