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In this paper, thick-section TC4 alloy was welded to itself by laser-MIG hybrid (LAMIG) welding. The microstructure of the welded joints was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results revealed that &#945;&#8217; and &#946; phases were formed in the weld seam. The effects of heat input (<i>E</i>) on the microstructure and tensile strength of the joints were investigated. With the increase of heat input, the residence time of platelet &#945;&#8217; martensite in the high-temperature phase changing zone became longer and the thickness of platelet &#945;&#8217; martensite increased. Furthermore, the &#946; thickness became large, and tangling dislocations were found to exist in platelet &#945;&#8217;. In addition, the increasing heat input could cause a decrease in the tensile strength. The failed dimple pattern experienced a change from equiaxed to tearing, which was harmful to the property of the joint.