A combined preheating and oscillating arc process was carried out in narrow-groove butt joint welding to produce a defect-free weld joint containing a thick aluminum alloy section. The weld bead formation, welding defects such as lack of fusion and porosity, and the angular distortion of the weld joint were studied at various preheating temperatures in both a systematic experiment and a theoretical analysis. The microstructure characteristics of the weld joints produced at varying preheating temperatures were analyzed. The morphology of the β phase and its segregation behavior were observed in the region close to the lack of fusion and inter-pass of the weld deposit. The tensile strength of a sound welded joint with proper preheating reached 90% that of the base metal.