Abstract: A simulation test of Ti-6Al-4V titanium alloy was made by means of the welding thermal simulation technique. The microstructure and phase composition characteristics of base metal and thermal simulation samples in different cooling rates were studied with optical microscope, scanning electron microscopy and X-ray diffraction analysis. The result demonstrates that the lamellar α is growth along the β grain boundary under the low cooling rate. When the cooling rate is increased, the acicular α' martensite is formed along with the α-phase reduction. XRD analysis indicates that by comparing the thermal simulation specimens to the base metal, the diffraction peaks of the hexagonal close packed crystal α and α' phases are subjected to some erosion. The microhardness increases with the increase of cooling rate, but the effect is not obvious.