By the 3D coupled thermo-mechanical finite element model of linear friction welded TC4 alloy, the relations among the temperature, the axial shortening and the flashes are researched.The numerical simulation results show that the flashes with the wrinkles appear only when the temperature of material in the friction surface is high enough.The flashes, which are made up of material with high temperature, are squeezed out of the component and then make the temperature increase slowly, even decrease.Compared with the oscillation frequency and the oscillation amplitude, the friction pressures increase can more easily attain the flashes with the bigger dimensions.With the increase of forging pressure in the forging stage, the axial shortening increases and then the decreasing velocity of temperature of component increases.Moreover, during the linear friction welding process, the increase of temperature is good for the increase of axial shortening of component, which influences the dimensions of flashes.