Owing to the lack of matching commercial welding wires, the development of wire arc additive manufacturing (WAAM) for most aluminum alloys is hindered. A wire-powder synchronous arc additive manufacturing (WPAAM) was proposed to prepare the target Al-Si-Mg aluminum alloy. Based on the synchronous deposition of AlSi₁₂ wire and pure Mg powder, the deposition width of the WPAAMed thin-wall was increased by 61% compared with that of WAAMed thin-wall using AlSi₁₂ wire, and the machining allowance was reduced by 81%. The added Mg powder benefited to form refined equiaxed grains, and reduced the average grain size of the WPAAMed thin-wall to 47.1 μm, showing a decrease of 23.8% relatively to that of the WAAMed thin-wall. Besides, Mg reacted with Si to form Mg₂Si strengthening phases. The mechanical properties tests showed that the ultimate tensile strength and elongation of the WPAAMed thin-wall increased up to 174.5 MPa and 4.1%, reaching 92% and 60% those of the WAAMed thin-wall, respectively.