Linear friction welding (LFW) of Ti6Al4V titanium alloy was simulated using 3D coupled thermo-mechanical finite element method. Effects of welding parameters (including oscillation frequency, oscillation amplitude, friction pressure, and forging pressure) on material flow behavior were investigated. Results of numerical simulation show that material flow velocity increases with increasing friction time and decreases with increasing forging time. Material flow velocity increases, and more material flows toward the edges of friction surface with increasing oscillation frequency, oscillation amplitude, and friction pressure at the friction stage. A reasonable axial shortening is beneficial to extrude the oxide out of deformable component. When the dimension of friction surface is 11.8 mm perpendicular to the oscillation direction and the oscillation amplitude is less than 2.5 mm, the axial shortenings at the friction stage and at the forging stage are respectively bigger than 4 and 0.65 mm in order to achieve the self-cleaning mechanism of LFW.