Abstract: The TC4 titanium alloy plates with 45 mm thickness was welded by using double-sided submerged arc welding method, and then obtained the defect-free butt joint. The microstructure of welded joint was observed by optical microscopy. The results show that the microstructure of weld seam was β columnar grains, which consisted of basket-like α′-martensite. The lath α′-martensite was formed because the effect of the later weld seam on prior weld seam. The acicular α′ martensite was formed for a short residence time for lacter weld seam at high temperature; HAZ comprised part recrystal zone, transition zone, fine grain zone and coarse grain zone, the grains in HAZ were equiaxed but with different sizes and forms. The part recrystal zone consisted of equiaxed structure of α and β, transition zone consisted of equiaxed α and acicular α′ martensite and remained β grain; fine grain zone consisted of acicular α′ martensite and remain β grain, coarse-grain zone consisted of basket-like α′ and remained β grain.